The basis of retirement income is reported in Box 9b of Form 1099-R.
If nothing is reported in Box 9b, check to see if the amount is the same in Box 1 and Box 2a. If so, then you have no basis (after-tax contributions), and your basis is zero.
If Box 2a is blank and Box 2b is checked, then you are going to rely on your memory and your records.
For a traditional IRA, you would have a basis if you made after-tax contributions. Those would be reported on form 8606. If you don't recall making any after-tax contributions, or if your IRA consists entirely of rollovers from 401k or 403b accounts, then your basis is zero.
If this was an annuity you purchased on your own, your basis is the amount you paid for it.
For all other circumstances, you will have to check your records to see if you made after-tax contributions to the plan.
