How do you prove you are the executor of estate and need to cash an insurance refund check?

When you were appointed the executor the court issued "Letters Testamentary" in your name. That is your legal authority to represent the estate and sign any checks made payable to the estate or to the decedent. You should bring a copy to the bank where the decedent had their account and speak to the manager about cashing the check. The bank may put a hold on the funds until the check clears. Once you are certain there will be no more checks issued to the estate you can then close the account.

If the decedent had no bank account then you should seek advice from the attorney who is handling the estate. If you do not have "letters Testamentary" then you are not the duly appointed executor and have no right nor authority to cash the check.