Most listeners use a combination of both approaches. You can examine the FCC license on premise. I have found the actual FCC radio license, complete with frequency assignments, hanging on the walls of places like the mall security office or company guard shack. You can examine the labels on radio equipment. Frequency information is engraved on labels on the back of many walkie-talkies, or inside the battery compartment, like in the Motorola HT220 model.
Most pagers have labels on the bottom or inside.
Like passwords taped onto terminals, it's not uncommon to find Dymo tape labels embossed with frequencies or call letters glued to the front of base stations. If informa- tion is displayed publicly, then a reasonable person could assume it's not government secret. Hobbyists are urged to exercise a modicum of restraint and good judgement, how- ever. Most programmable scanners afford the abil- ity to search between two frequency limits set by the user. Three models, the ICOM R7000, Bearcat 250, and Regency K500, have the ability to automatically store active fre- quencies found during an unattended search operation.