How is a circuit for a fire alarm marked in the panel?

In the US, NFPA 72 (2007) 4.4.1.4.2.2: "Circuit disconnecting means shall have a red marking, shall be accessible only to authorized personnel, and shall be identified as 'FIRE ALARM CIRCUIT'".

Other codes, or earlier versions, may have slightly different requirements, such as marking it "FACP" for "fire alarm control panel."