The tax lien means only that the taxpayer owes tax, not that it was a criminal matter. Often these cases are publicized, and a Google search might turn it up. You would be smart not to work for that company again.
This is not intended to be legal advice, and is general in nature.
For federal you will need to search the records of the district court. Depending on where the subject lived you may need to search records at more than one district. For state taxes I believe you would search the criminal index for the relevant counties.
There is a big difference between a tax lien and tax fraud. The lien is issued as part of collecting taxes. Sometimes the numbers on a tax lien are huge because the tax payer failed to file a return, and they will be adjusted when the return is filed.
You will also want to be cautious about what you do with whatever information you develop. You can wind up with civil liability for invasion of privacy, defamation and/or a lawsuit to defend.
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