First, it is important to note that you can change how property is characterized before of during the marriage (See Ca Fam § 850(a),(b) & (c) and Ca Fam § 1500. It states that spouse's statutory property rights may be altered by premarital agreement or marital property agreement). When it comes to how property is characterized, intent is key here. I would argue that property one spouse gives to the other spouse, with the intent to allow the other spouse to exercise full and uninterrupted dominion and control would be the separate personal property of the spouse receiving the gift. However, under Ca Fam § 2640(c), "a party shall be reimbursed for the party's separate property contributions to the acquisition of property of the other spouse's separate property estate during the marriage, unless there has been a transmutation in writing pursuant to Chapter 5..." In other words, if one spouse gave the other spouse a gift during the marriage and the couple divorces, then the spouse who gave the gift can seek reimbursement by the spouse who received the gift, unless there is something in writing stating that the spouse who gave the gift intended that the property intended that the gift be characterized differently (e.g. separate property instead of community property).
However, if money has been placed in a joint checking account, I would argue that it would depend on the purpose of the account. If the purpose of the account was to pay shared expenses, then absent an agreement to the contrary, the account could be characterized as community (as opposed to separate) property. The legal basis for this is that there is a mixture of funds in the account and the fact that the funds were generated during the marriage.
Intent is the main factor in both cases and much of the evidence would hinge on proof of the intent of the parties (e.g. a written agreement, circumstances, expectations of the parties, etc.)
I hope this helps.