I want to confirm if my 2018 returns had 401 k contribution deducted from taxable income?

[SweetieJean]

Were they listed on your W-2 form?

[VolvoGirl]

401K Contributions come out of your paychecks pretax meaning before taxes and have already been deducted from the wages in box 1 of your W2 and you can't deduct them again.  You only enter them from the W2 box 12 nowhere else,  

[Opus 17]

If you don’t know whether your gross income (box 1) was already reduced by the 401k contribution you will need to ask your employer.