How can I prove my spouse withdrew money from his 401k while in the process of divorce but he blamed me

There are forms that must be filled out to withdraw money from the account. You would likely not be able to access his account without his permission. Proof of his signature as well as other documents from the fund manager would likely suffice to show that he gave authorization. Also, the more important issue would probably be to trace what happened after the money was withdrawn. For example, if the money was used to take care of the household, it may not matter to the court. But if the money was used for one spouses benefit, the court may use that to equalize things in the property division. If you both have attorneys they will certainly make arguments concerning the withdrawal of the funds.