You don't determine the taxable amount. Just enter the 1099-R into TurboTax and answer the follow-up questions. The taxability is dependent on the code in Box 7 and your answers to the follow-up questions. Code G is used for a direct rollover and entering a code G will make the distribution non-taxable.
If your form 1099-R does not have a code G, the distribution will initially be taxable. However, when you answer the follow-up questions and if you indicate that you rolled the amount over within 60 days, TurboTax will adjust accordingly. Make sure you go all the way through the retirement section after entering the 1099-R.