Other times, the frequency information is there for the taking - without hassle. Monitor a frequency or frequencies, and try to determine who's transmitting and what purpose the channel serves. Once you identify the user, log the information. Readers are urged to abide by the rules of good taste and local laws in the quest for frequency information. Don't trespass, wait for an invitation.
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.