The only way to know if there was a basis would be from old tax returns showing the 8606. Without that form, you can't prove basis. You can get a tax transcript and see if there is an 8606 in there. It will also show the distributions and tax return so you can see if there is a difference. Most people do not have a basis, but some do. See About Form 4506-T, Request for Transcript of Tax Return
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