How do I tell / where do I look to find out if there's a lien on my mortgage from city fines acquired years back that were thought to have been paid off.

You check the County Clerk it will be listed in the county deed records.

Actually the lien is not on the mortgage but on or against the property. Any record of this would be at he county office where the recorder of deeds office is ( or similar name in your area) You will have to review these records to see if there was a lien placed against the property