I would like to know how to find the IL tax withholding/payments for my prior year taxes

one source is your w-2s. For 2021 it will appear on line 25.  forms change from year to year so you'll need to read the IL-1040 to see for other years. 

Illinois state tax withheld is on line 25 of IL-1040 as Champ @Mike921 said. If you made state estimated tax payments, that's on line 26. 

 

If you owed any tax from 2020 or other years, you would include that amount in withholding/payments for prior year taxes.