Ryanair and Easyjet .... Anyone can sit at the overwing for nothing and the crew can do very little about it without causing a riot.
Not the case at all. Both have an FA on duty in the centre of the aircraft alongside the emergency exits during boarding who tells you as you take these seats what they are, checks you are within the parameters, and proceeds to give you the emergency exit briefing. So no riot.
Ryanair has always used distinctive yellow headrests on these seats which makes them stand out, far more than the emergency floor lighting changing colour from white to red at the exits - other airlines please note.
Noticeably these seats tend to be taken early on by what look like "regulars" making for them.