How many years of prior tax returns am I responsible for when paying taxes for a deceased person who's estate is in probate?

If the debts existed at the time of the person's death they are debts of the estate. I would request income tax return and account transcript information for the deceased to see what you are dealing with. You also say there "does not seem" to be a lien - what do you base this one? I would advise running a lien and judgment search in the county where the decedent owned real estate.

The PR's liability for a decedent's taxes runs essentially to every tax year that the decedent did not properly pay all taxes owing, AND where the statute of limitations hasn't already passed. The IRS has three years from date of filing to challenge a return, so that is reassuring for any year the decedent filed a return. BUT if the decedent did not file a return, the statute of limitation never runs. So depending on the circumstance, I often advise a PR client to request records from IRS to confirm that returns actually were filed--and if not, then have the PR prepare and file them (or hire an accountant to do it).

If you don't pay your taxes there is no statute of limitations as far as I know. Hence, you have to pay all prior tax debts not paid by the decedent but, obviously, from the estate of the person. Your probate lawyer should be able to answer this for you and if you don't have one it seems pretty clear that you should have one.