We've had to do this in VA for a long time.  Someone with a "Fairfax" address or zip code might live in either Fairfax County or the City of Fairfax (each has their own locality code).  In VA we also have personal property taxes at the locality level.  So if they paid their car tax to the City of Fairfax instead of the County of Fairfax that's a pretty good clue.

For new clients (or clients who move) I used to use usps.com (now I don't have to, Drake populates this automatically based on the address).  It's gotten harder to find on their website but you can look up a zip code by address:

https://tools.usps.com/zip-code-lookup.htm?byaddress

On the results page there's an arrow that gives you more details including the locality.