At United, you check in with the gate agent, show your airline pilot ID and license. The gate agent then enters your information into the computer and a jumpseat boarding pass with your name printed on it is issued. Your final approval is subject to the captain's review and consent. You do not have to be in uniform, only suitably dressed in collared shirt, slacks, etc. Since the attack, only United pilots may jump. I frequently jumped on United though I am from another carrier (thanks, UA). No student pilots, interested people, or others without expressly authorized status may jumpseat.