His forging your signature on the check is definitely illegal.
While it wouldn't be considered illegal in the past, before the separation, it is since the separation.
So you have 3 options now - you can do individually or all together (potentially - #3 may be a problem if the court determines that the Family Court has jurisdiction in the matter which is usually what they will do under the circumstances):
1) file criminal
charges with the District Attorney's office in the matter, and
2) file a motion for contempt and sanctions against him with the Family Court - that he wrongfully forged your signature without your knowledge and consent and stole your monies. the judge would at least sanction him and mandate that he turn over the monies and pay your attorney's fees
3) file suit in civil court for 1/2 the monies - but usually they will dismiss the action and state that the Family Court has jurisdiction over the dispute.
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