New transmitters come with a red flag designating the radio as being on an aircraft frequency (72 mHz band). They also come with the frequency number (black lettering on white flags or placards) which must be displayed on the antenna (AMA required). If you bought a used radio without these items, they are available at your LHS. The TX crystal should have the frequency marked on it, and sometimes the radio itself has a small label denoting the frequency. Most clubs use a card/pin system. You take the proper frequency pin from the rack and replace it with your AMA or club membership card. Some clubs are slack about this, and I'd be REALLY careful in that situation.

Dr.1