To get the IRA Deduction Worksheet in TurboTax, you'll need to enter a proposed traditional IRA contribution. TurboTax will then inform you of the deductible amount of your proposed traditional IRA contribution. You can then go back an modify the proposed amount to be the actual amount that you intend to contribute. With a proposed IRA contribution entered, you can then review the IRA Information Worksheet to see the MAGI that TurboTax calculated. (Nowhere can you actually view the calculation of MAGI.)
You can calculate MAGI yourself using Worksheet 1-1 of IRS Pub 590-A:
https://www.irs.gov/pub/irs-pdf/p590a.pdf
