I was cashed out of a 401k last year, and I believe they took taxes out of that check. But I'm not sure what form I need or where to insert that information on this site.

[Opus 17 · Accepted solution]

You should have received a form 1099-R from the plan administrator.  If not, call them or check your account online.  Enter the 1099-R under the section for Retirement Income.

Your total tax owed will depend on your other income, deductions, and whether you are exempt from the penalty or not.  You will get credit for the amount withheld from the distributions (along with any other tax payments or job withholdings) and the final computation will show if you owe more or get a refund.