Over-wing Exits
 

Airlines carefully check the size of carry-on bags to make sure that they will fit in the overhead bins.
How do they check the size of passengers to make sure that they could fit through the emergency over-wing exits?

They don't. Doors are designed to meet certification specification and nothing more. Additionally, you have main cabin doors for egress for anyone of size.

You won't get anyone too big sat next to them on most airlines as extension seatbelts are not permitted at exit rows. That's not to say people from rows other an an exit row won't try to use the exit but let's face it, there are many people for whom over wing exits are not suitable for, both in terms of getting through the exit and then getting down to the ground.

Let me guess what has happened here. You're slim, you've been pulled for tour bag being too heavy or too large and you think it in unfair that people weigh more than you don't have to pay extra?

Luckily, my bag passed the size check.
But I did wonder how the cabin crew deter large persons from attempting to use, and possibly blocking, the over-wing exits in the event of an emergency evacuation.

Cabin crew usually man main cabin doors. Over wing exits (of the hatch type) are usually self help and the cabin crew do not operate nor do they supervise evacuations through the over wing exits. So there's nothing to stop someone trying to get through but I should imagine most people of size who May feel like they won't fit will not go near the over wing exits as they wont want to get stuck in a potential burning aircraft.

The size of people is not the issue. Taking cabin bags through over wing exits is what will kill people, not an overweight person.

They have an enormous plunger - about 3ft across - that's hidden in the overhead locker - apply to blocking body and one good heave and they go out like Auric Goldfinger

I'd imagine following passengers would make sure they got out - cork out a champagne bottle springs to mind!! :-)

Or perhaps the problem doesn't arise because the slower-moving fatties get trampled in the rush.