They'd never check, even if they knew you were high. They'd just check for possession.

Look I don't live in the USA but I've watched Boston Legal a lot and I can say that it would be impossible for them to stop and bust you for 'being high'. The only way to prove it in court would be to take urine/blood tests to prove that THC is in your system. That takes a warrant, which they wouldn't get because there is not enough circumstancial evidence to say that you are in fact high. Even if they did get a warrant, a good lawyer could delay the tests as long as it would take. The police wouldn't want the paperwork for all this shit.

Simple fact is the only way they can hold you (and get a warrant for that matter) is on probable cause of being under the influence, as Myth1184 has said. So just be cool and don't worry about it. Enjoy your flight.