For transit buses, you can usually just look up the number in a roster if you know what transit agency runs it.

Idk about government agencies, but contacting the company does work.
There was a crane in Brainiac that we couldnt identify. I contacted the company "Lee Lifting" (there site even says that they have appeared on that show) and i got a reply back with the make and model.
It's a Kato NK500-EV. So if someone could please change that.
http://imcdb.org/vehicle_204435-Unknown.html

How often have the rest of you written to government agencies in the effort to identify vehicles? I recently realized that after getting no help from Hank's Truck Pictures, I had no other choice but to make a last-ditch effort to identify an NYC Sanitiation truck in "Up the Down Staircase," by writing to the Department of Sanitiation itself.

http://www.imcdb.org/vehicle_180522-unknown.html

Also, has anybody heard from San Francisco MUNI about the trolley bus in "The Princess Diaries," yet?
http://www.imcdb.org/vehicle_84384-unknown.html