Generally, at the time of divorce, the court is able to make determinations with respect to such situation. If the money was withdrawn, your spouse is likely to be obligated to repay your portion to you. Please note, until there is a divorce action pending and your spouse has been directed to preserve the property, you have no way of stopping her. While you mention that you are in the process of separating, until there is a valid separation agreement or judgment of separation, you do not have any rights in your spouse's retirement funds, and even then, they are not likely to be segregated until there is a judgment of divorce pursuant to a Qualified Domestic Relations Order. The issues of proof are not significant here since the 401k fund is able to provide information with respect to the value of your spouse's interest and amount withdrawn.
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