There's not a worksheet or similar document that shows the MAGI calculation.
If you're on a desktop version of TurboTax, you can see Form 8962 if you'll enable the Forms mode in the upper-right corner of the program. However, this won't show a MAGI breakdown.
MAGI, for Premium Tax Credit purposes, is based on the information you enter in the TurboTax program. You can see what's taken into account for the calculation in Worksheet 1-1, Taxpayer's Modified AGI—Line 2a and Worksheet 1-2. Dependents' Combined Modified AGI—Line 2b in the Instructions for Form 8962.
