When I fly non rev I try to keep my head down and not let anyone know I work for DL. I have noticed that certain people are not as discreet (buddy passes and new hires especially)

If you are late to board you can often find nonrevs hanging around but not directly in front of the podium in hopes of hearing their name and running down the jetway. I've litterally had gate agents walk me down the jetway to physically look for a seat open. Then I sit down and pray for the door to close.

I've been pulled off of planes that I was sitting on when a mechanical hit another flight and they reaccomidated people on the plane I was on. About 10 non rev were pulled off that flight and all went quietly and quickly with no fuss or discussion. That is (hopefully) another big way to spot a nonrev over compliance with crew/GA instruction with no complaints where you would expect paying passengers to pitch a fit.

I hope this helps