If you've been using TurboTax throughout, each time you entered a nondeductible traditional IRA contribution , TurboTax would have prepared Form 8606 Part I to report that contribution and TurboTax would have carried to the IRA Information the result on line 14 of that form to be added to the basis from prior years already on that worksheet. After entering a traditional IRA contribution or entering a Form 1099-R that reports a traditional IRA distribution, TurboTax will present the question that asks if you made any nondeductible contributions for prior years and, if you answer Yes, will ask you to enter or confirm your basis.
