@LMTaxBreaker
The best proof would be copies of form 5498 that is issued to you by the IRA every year (usually in May) which documents the prior year contributions. The IRS also gets copies of this form, and you can check your online transcript, but that only goes back 7 or 10 years. If you don't have form 5498 and they are too old to get from the IRS or the account custodian, the IRS may be willing to look at other contemporaneous written records -- that means a written record that is made close enough in time to the event that it is likely to be accurate (and not just a spreadsheet you prepared from memory the night before the audit). If you had used Turbotax every year to track the contributions, there is no guarantee the IRS would accept it, since you can enter anything into the program that you want whether true or false (they could still ask for backup proof to match the program entries).
https://www.irs.gov/individuals/get-transcript
The laws and court rulings basically state that all income is presumed taxable, and the taxpayer is not entitled to any deduction, reduction or adjustment that they can't prove. I would start by getting as many of your old records together as you can.
