When did you cash it out? If it was in 2015 you should have received a 1099R for it that will show the withholding taken out. And you should have got some paperwork or a check stub with the payout that shows the deductions. When you asked the plan to take it out you filled in the sheet and told them what percent to take out for federal and state taxes or they take out a set percentage like 10 or 20%. If you can't find all the paperwork ask the plan for the details.
