Hi and welcome to Just Answer!The IRS has to accept any documentary proof for your statements. If you provide copies of cancelled checks with your ex-spouse signature - that are a perfect supporting documents.We may not say for sure if the IRS finds these documents adequate to support your statement that this bank account actually belongs to your ex-spouse. I suggest to contact the bank directly via certified mail and attach a copy of the IRS request - asking them to respond directly to the IRS. If the IRS finds your supporting documents not adequate - you will provide a copy of your communication with the bank.Most likely the bank will ask that the IRS would contact then directly - and you will forward a copy of that letter to the IRS. Thus - if they still have doubts - they have capacity to verify that.