To know your basis in Roth IRA contributions and conversions, you'll need to reconstruct the contribution, conversion and distribution history of your Roth IRAs. All of the necessary information should be captured in Forms 5498 and 1099-R issued to you and the IRS for these transactions in your Roth IRAs, but you can corroborate that with information from account statements and tax returns. Forms 5498 and 1099-R are captured in your Wage & Income transcripts available from the IRS.
