So, after filing my taxes, I made a discovery that my now former employer was committing tax fraud. What do I need to do before contacting the IRS?

My former employer was taking out state taxes from each paycheck, however he was NOT depositing the money to the state income tax account. I extrapolated the numbers based off recent (2021) paystubs (assuming he did not change my W4 information behind my back without my knowledge) and I've determined he was definitely taking the money out. However, I ended up only paying a limited amount of state taxes according to my W2.  However the money came out of my checks. I did not claim unemployment at any point during the coronavirus pandemic, and claim no dependents.
What do I need to do before contacting the IRS and reporting him? I do not have any paystubs from 2020 as he did mostly direct deposit and always dragged his feet about me asking for pay stubs. I am employed and live in Indiana.