How do ski lift operators check to see if there aren't anymore people on them?

At the end of the day, they radio the operator in the shack at the top the number of the chair of the last person who got on. Then the lower operator closes the gate that admits more riders. When the last person gets off, they shut it down for the night.

Every once in a while someone gets stuck on a ski lift overnight, and it’s often with tragic consequences. http://newschannel9.com/news/local/lawsuit-claims-chattanooga-boy-stranded-on-nc-ski-lift-overnight

http://www.nbc11news.com/home/headlines/Lost-skier-and-snowboarder-rescued-after-overnight-in-Bear-Creek-286887211.html

There is a seat number on he back of each chair. They check the number of the last person allowed on; when that number comes back around. they know the whole lift has cleared.

And, as @snowberry pointed out, they communicate with the operator at the top of the mountain. The top operator is also checking the numbers.

The ski patrol also checks, although they cannot always get underneath every chair.

Also, when the guys who maintain the lift drive down in the snowmobile, they look up and do a visual check as a double check.

At a given time they stop allowing people to board the lifts at the bottom and communicate this to to operator at the top as @snowberry notes. The lift may keep going for a while because they also use it to transport employees back down the hill but no one gets on at the bottom. And although accidents occur, usually several people will ski down the lift tracks to verify that no one remains on the lifts.

@snowberry In your first link the stranded person was referred to as a “boy.” It would be interesting to know exactly how old, and why his parents didn’t notice that he wasn’t with them.

I googled it and it says the lawsuit doesn’t list his age. There are photos of him, though.

http://www.charlotteobserver.com/news/local/article135568343.html

It says here he’s 14 (I’m the Queen of the Google).

https://www.clarionledger.com/story/news/local/2017/03/03/youth-hurt-jumping-stalled-ski-lift-sues-wnc-resort/98685984/

Lots of children ski alone, especially if they’re advanced skiers and they go faster than everyone else. It’s always been a pretty safe environment. Not many people could have caught me when I was a kid! LOL

Well, this was after the lift and the ski slope closed down for the day. He was stuck there several hours. I’d be concerned, especially as the sun started going down, I don’t care how old he was.
In one of the links @jca posted above it says the parents reported him missing but the authorities just kind of blew it off.
The ski patrol, however, said they started looking immediately.