An addendum:

In terms of a regional transit bus. If you don't use a mirror, which would give you the chance to see the destination listed, since they were going the same direction. All you would have to do is remember the route number from the back of the bus. The direction you were traveling(ex. south on Lombard St. in San Francisco), at the time, and the general time of day.

Since you think it would be an express bus. That would mean during AM or PM-Rush. If it was an 'off-duty' regional transit bus. It would say 'Not In Service', on the electronic sign on the front. Also, The express busses will generally take different routes than the regular routes. They may use the local interstate on a consistent basis.

Was it the shape of a:

regional transit bus: long, rectangular, no curved edges on top

School bus: long, curved edges on top, some with engine in front of driver, some with engine underneath driver; short, curved edges on top, engine in front of driver

Church/DSS bus: short, curved edges on top, engine in front of driver