If you are asking if you can use your 2018 W-2 box 1 income for your AGI to file your 2019 tax return, you can try it. However, there is no way to know for sure that it was your AGI as shown on your 2018 tax return.
Here are ways to get your AGI: https://ttlc.intuit.com/questions/1900840-how-do-i-find-last-year-s-agi