Easiest way: pull last year's Form 1040 AGI is on Line 11. If you don't have a copy, options: (1) IRS "Get Transcript" online at irs.gov (instant if you can verify identity) or by mail (5-10 days), (2) if you used tax software (TurboTax, H&R Block etc.), log into that account and pull the prior return, (3) call IRS at 800-908-9946 to request a transcript by mail. If you're e-filing this year and need last year's AGI to verify identity, the transcript is your fastest bet.