How do flight attendants know there are cell phones still on

Probably don't. Bluffing to make everyone shut them down.?

They don't. I've been on plenty of flight where people don't power their phone off. Just turn them upside down on their lap. The flight attendant never says anything and we take off anyway.

My DH is a pilot and he confirmed that there isn't any way to know if a phone is on. If there is a cell phone left on very close to the pilots' radio, there is minimal interference but it would have to be inside the cockpit to cause that interference.

She was bluffing - unless she saw someone's phone on.

I'm guessing that she (or another flight attendant) saw the phone. That would be the only way that she could know.

They are just bluffing, as others have said. But them saying it will cause at least one more person to look down and switch to airplane mode.

She's either bluffing, or she could see someone on the phone refusing to stop. Otherwise you can't tell. And now, you just have to turn it to airplane mode and it's good.

I'm guessing she was probably being a smart ass to get the attention of the one jerk who wouldn't get off their phone for takeoff.

My close friend is a flight attendant for AA. I asked her once why we really had to power off electronic equipment and she said it was so that people would pay better attention to the safety messages and be more alert and aware during take off, landing, and when plane is on route to runway.. She said that's when most accidents happen.

There is always somebody with there saw phone on! Just like there's always somebody who does not have their seatbelt fastened!

It's pretty obvious when people are looking at their crotches and smiling

Oh, sorry, I didn't see your post before I posted, so it looks like I was "responding" to you when I was just responding in general! It was exactly as @carblover443 describes.

A good friend of mine is a flight attendant so I asked. He flies with a smaller airline and they do ask for them to be off. Here is his response.

Sometimes when someone uses a cell phone the pilots hear static over their radio it has the possibility to mess up communication between the tower and the cockpit, not always, but sometimes. Plus depending on the situation like things called Cat 3 landings which is complete instrument landing with no visibility, they make everyone turn off all electronics. The plane's instruments are so sensitive they can detect anything. It won't make a plane crash, but it can be a distraction.
Planes just don't fall out of the air because of cell phones etc... but I think I would rather have the pilots have no distractions at all.