Passengers who are in a bad mood.
"If people are having a bad day, I definitely notice that," the United flight attendant said. "You can tell by the person's body language or facial expressions or if they've been crying. I know that air travel is really hard for people."
What the "ding" sounds mean.
"There's a lot of 'dings' that go on on the plane that mean specific things to the flight crew, but passengers don't know what any of them are," a flight attendant for Delta Air Lines said.
There are "dings" for phone calls, when the plane has reached an altitude of 10,000 feet, and when the seatbelt sign is turned on or off, among other events, she said.
Which passengers need the most help.
"I'm noticing the passenger that's a single mom with their kid that might just need a little bit more help or a just a little bit more patience," a flight attendant for Delta said. "I'm noticing the passenger that's clenching onto their armrest really hard that's really freaked out about flying and probably needs somebody to talk to."
The location of the nearest exit.
"I know where my closest exit is," said Lauren Redling, a flight attendant for the private-jet charter company Worldwide Jet.
Which passengers are the most anxious.
"I can tell an anxious flyer from a mile away," said Kevin Cain, a flight attendant for PSA Airlines. "A free glass of wine can just calm their nerves and change their day around."
When passengers try to keep their bags in their laps instead of stowing them under the seat in front of them
"I always notice if they're trying to hold their bag in their lap — always," said a flight attendant for Envoy Air. "They can try to hide it from me. I always notice."
Where the bathroom is located.
"I, for one, always know where the lavatory is, whereas the average passenger doesn't," the Envoy Air flight attendant said.
Which parts of the plane are dirty.
"Passengers seem to not think that anything is dirty on a plane, which is just funny," said Sally Ann MacLagan a flight attendant for Mesa Airlines.