Legal Question in Wills and Trusts in Illinois

An individual would have to prove he or she had a right to the information before any financial institution would release it. Usually, the surviving joint tenant will have a right to the information. Now is the time of year tax statements are being issued, a review of the decedent's mail would help identify the existence of any accounts, so would a review of the deceased person's last income tax return. There should be a mail forwarding order filed with the post office by the individual with appropriate authority to make the request.

I would add that your question needs to be reversed. It is presumed that the accounts were established legally and you would have to rebut that presumption by casting some doubt on the establishment of the accounts (such as undue influence, forgery, incompetence to understand the act of establishing the joint account). So, the question is really about proving that the accounts were established "illegally."