1. For traditional deductible IRAs :
Look at your past tax returns. If you contributed to a deductible IRA you should have claimed a deduction for them

2. For traditional non-deductible IRA contributions:
You should have filed Form 8606.

3. For ROTH IRA contributions:
There is no form on your tax return. Your financial institution should have the info. A ROTH IRA should be a separate account from your traditional IRA(s).

4. Despite instructions otherwise, I would keep ALL my tax return records, particularly Form 8606 for non-deductible IRAs, because this is the only way I know of to prove that the portion has already been taxed.