If he did take his share out, he should have given you a statement itemizing what his share was and then your net. If you have that, you can introduce it as evidence that you already paid the attorney.
Or, if your divorce decree states an amount you are supposed to get and your check was less than that amount, you can introduce your check as evidence that some went to the attorney.
Likewise, if your ex's attorney's check to your attorney was for more than your attorney's check to you, that would be proof of how much your attorney kept. You can subpoena the other attorney's check to prove this point. You can also subpoena your attorney's file to see if there is anything in there that will prove that you've already paid (likely there won't be, or else the attorney wouldn't have a case).
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