When you enter the 1099-R for the distribution, TurboTax will ask about prior contributions and then about prior conversions. A distribution of earnings from a conversion done less than 5 years before the distribution might be taxable but after 5 years is not. TurboTax will ask the amount of conversion and the year of the conversion to calculate that.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**