9.3 Operators should also consider providing an abbreviated briefing to passengers
seated at self-help emergency exits in order to determine their suitability to occupy
those seats, seek their initial agreement to assist in an emergency and to advise them
to read and understand the operating instructions contained on the seat-back
placards, exit hatch and safety card.
Sure, ask the PAX. If they are smart, they'll know what the answers should be in order to qualify for that extra legroom. But does the cabin crew size them up as to ability?
I've seen people seated by the over-wing exit that look like they are bigger than the door plug. Or too gangly to get through the opening. But either of these types are exactly who the FAs often feel sympathy for their need for the extra space.
Seat a 5'-8" (1.73 m) ex-marine next to the door and watch all the tall people grumble for the duration of the flight.