This seems like one of those problems that can be solved realtively easily with crowd-sourcing. A kind of "verified purchase' but in real life. Open your Google Maps app while standing in the law firm's offices - and click 'yes I can see them' Maybe a photo.

Tie it perhaps to a UUID sent to the business owner (similar to adding a UUID to your DNS TXT record)

And even the reverse is possible, I could vouch that there is no law firm at this location - if I owned the building I probably would want to ensure that it only had registered businesses there.