I can't speak for other airlines, but with my airline you cannot pre-book or pre-reserve exit row seating without being made aware of the requirements of sitting in those seats - and your suitability to sit there is checked by staff upon arrival at the airport (you cannot self-check-in - it must be done by a check-in agent). In the event that, for some reason or another, an unsuitable person has been seated in those rows (it does happen sometimes
) they must be reseated before the doors are closed - I wont close the final door (I am a Cabin Supervisor) until I am satisfied that pax seated in exit rows are suitable - I am generally informed of any inconsistencies by the crew allocated to provide the specialised briefing to those pax.