A spouse cannot sign another spouse's name, as that constitutes fraud (marriage does not negate this). As for a joint account, both account holders have a equal right to the money in the account. To deposit a check, if the name on the check is also the name on the account, the bank will accept it for deposit (signature is only required for cashing, versus depositing). However, each spouse owes the other a fiduciary duty per the Family Code statute located below:
721. (a) Subject to subdivision (b), either spouse may enter into any transaction with the other, or with any other person, respecting property, which either might if unmarried. (b) Except as provided in Sections 143, 144, 146, 16040, and 16047 of the Probate Code, in transactions between themselves, spouses are subject to the general rules governing fiduciary relationships that control the actions of persons occupying confidential relations with each other. This confidential relationship imposes a duty of the highest good faith and fair dealing on each spouse, and neither shall take any unfair advantage of the other. This confidential relationship is a fiduciary relationship subject to the same rights and duties of nonmarital business partners, as provided in Sections 16403, 16404, and 16503 of the Corporations Code, including, but not limited to, the following: (1) Providing each spouse access at all times to any books kept regarding a transaction for the purposes of inspection and copying. (2) Rendering upon request, true and full information of all things affecting any transaction that concerns the community property. Nothing in this section is intended to impose a duty for either spouse to keep detailed books and records of community property transactions. (3) Accounting to the spouse, and holding as a trustee, any benefit or profit derived from any transaction by one spouse without the consent of the other spouse that concerns the community property.
So if one spouse deposits, in a joint account, the separate property of the other, without their permission, that spouse can be court ordered to reimburse the money.
If a divorce is impending, the best way to ensure one's financial security is to file for temporary spousal support and alimony. Many attorneys will either do a Family Law Real Property Lien, or will defer collecting payment until after the first hearing, when they request attorney fees and costs to be paid by the other spouse.
The authority for this is contained in the below statutes: (the idea is to "level the playing field") -
2030. (a) (1) In a proceeding for dissolution of marriage, nullity of marriage, or legal separation of the parties, and in any proceeding subsequent to entry of a related judgment, the court shall ensure that each party has access to legal representation, including access early in the proceedings, to preserve each party's rights by ordering, if necessary based on the income and needs assessments, one party, except a governmental entity, to pay to the other party, or to the other party's attorney, whatever amount is reasonably necessary for attorney's fees and for the cost of maintaining or defending the proceeding during the pendency of the proceeding. (2) When a request for attorney's fees and costs is made, the court shall make findings on whether an award of attorney's fees and costs under this section is appropriate, whether there is a disparity in access to funds to retain counsel, and whether one party is able to pay for legal representation of both parties. If the findings demonstrate disparity in access and ability to pay, the court shall make an order awarding attorney's fees and costs. A party who lacks the financial ability to hire an attorney may request, as an in pro per litigant, that the court order the other party, if that other party has the financial ability, to pay a reasonable amount to allow the unrepresented party to retain an attorney in a timely manner before proceedings in the matter go forward. (b) Attorney's fees and costs within this section may be awarded for legal services rendered or costs incurred before or after the commencement of the proceeding. (c) The court shall augment or modify the original award for attorney's fees and costs as may be reasonably necessary for the prosecution or defense of the proceeding, or any proceeding related thereto, including after any appeal has been concluded. (d) Any order requiring a party who is not the spouse of another party to the proceeding to pay attorney's fees or costs shall be limited to an amount reasonably necessary to maintain or defend the action on the issues relating to that party. (e) The Judicial Council shall, by January 1, 2012, adopt a statewide rule of court to implement this section and develop a form for the information that shall be submitted to the court to obtain an award of attorney's fees under this section. 2031. (a) (1) Except as provided in subdivision (b), during the pendency of a proceeding for dissolution of marriage, for nullity of marriage, for legal separation of the parties, or any proceeding subsequent to entry of a related judgment, an application for a temporary order making, augmenting, or modifying an award of attorney' s fees, including a reasonable retainer to hire an attorney, or costs or both shall be made by motion on notice or by an order to show cause. (2) The court shall rule on an application within 15 days of the hearing on the motion or order to show cause. (b) An order described in subdivision (a) may be made without notice by an oral motion in open court at either of the following times: (1) At the time of the hearing of the cause on the merits. (2) At any time before entry of judgment against a party whose default has been entered pursuant to Section 585 or 586 of the Code of Civil Procedure. The court shall rule on any motion made pursuant to this subdivision within 15 days and prior to the entry of any judgment. 2032. (a) The court may make an award of attorney's fees and costs under Section 2030 or 2031 where the making of the award, and the amount of the award, are just and reasonable under the relative circumstances of the respective parties. (b) In determining what is just and reasonable under the relative circumstances, the court shall take into consideration the need for the award to enable each party, to the extent practical, to have sufficient financial resources to present the party's case adequately, taking into consideration, to the extent relevant, the circumstances of the respective parties described in Section 4320. The fact that the party requesting an award of attorney's fees and costs has resources from which the party could pay the party's own attorney's fees and costs is not itself a bar to an order that the other party pay part or all of the fees and costs requested. Financial resources are only one factor for the court to consider in determining how to apportion the overall cost of the litigation equitably between the parties under their relative circumstances. (c) The court may order payment of an award of attorney's fees and costs from any type of property, whether community or separate, principal or income. (d) Either party may, at any time before the hearing of the cause on the merits, on noticed motion, request the court to make a finding that the case involves complex or substantial issues of fact or law related to property rights, visitation, custody, or support. Upon that finding, the court may in its discretion determine the appropriate, equitable allocation of attorney's fees, court costs, expert fees, and consultant fees between the parties. The court order may provide for the allocation of separate or community assets, security against these assets, and for payments from income or anticipated income of either party for the purpose described in this subdivision and for the benefit of one or both parties. Payments shall be authorized only on agreement of the parties or, in the absence thereof, by court order. The court may order that a referee be appointed pursuant to Section 639 of the Code of Civil Procedure to oversee the allocation of fees and costs. 2033. (a) Either party may encumber his or her interest in community real property to pay reasonable attorney's fees in order to retain or maintain legal counsel in a proceeding for dissolution of marriage, for nullity of marriage, or for legal separation of the parties. This encumbrance shall be known as a "family law attorney's real property lien" and attaches only to the encumbering party's interest in the community real property. (b) Notice of a family law attorney's real property lien shall be served either personally or on the other party's attorney of record at least 15 days before the encumbrance is recorded. This notice shall contain a declaration signed under penalty of perjury containing all of the following: (1) A full description of the real property. (2) The party's belief as to the fair market value of the property and documentation supporting that belief. (3) Encumbrances on the property as of the date of the declaration. (4) A list of community assets and liabilities and their estimated values as of the date of the declaration. (5) The amount of the family law attorney's real property lien. (c) The nonencumbering party may file an ex parte objection to the family law attorney's real property lien. The objection shall include a request to stay the recordation until further notice of the court and shall contain a copy of the notice received. The objection shall also include a declaration signed under penalty of perjury as to all of the following: (1) Specific objections to the family law attorney's real property lien and to the specific items in the notice. (2) The objector's belief as to the appropriate items or value and any documentation supporting that belief. (3) A declaration specifically stating why recordation of the encumbrance at this time would likely result in an unequal division of property or would otherwise be unjust under the circumstances of the case. (d) Except as otherwise provided by this section, general procedural rules regarding ex parte motions apply. (e) An attorney for whom a family law attorney's real property lien is obtained shall comply with Rule 3-300 of the Rules of Professional Conduct of the State Bar of California.