If a portion of your distribution is non-taxable because you made some after-tax contributions to your retirement account, that should be reflected on the 1099-R that you receive. Box 1 should contain the gross distribution while Box 2a should say the taxable amount of that distribution.
If the taxable amount not determined is checked, TurboTax will ask some follow-up questions in order to determine the taxable amount. In order to answer these, you will need to know about non-deductible contributions you have made if it is from an IRA.
