Spot a stranger on what you think is your seat? Double-check your own BP and the current row number first. If your seat is taken, kindly mention to the poacher that it's my seat, please, is their possibly a row in front or back? The day will come when I get temporarily stuck with this but so far it's always worked out neatly.
No, No, No. Best not to get directly involved. The day will come …….
Edit: Sorting out where pax should/ should not be seated is not our responsibility? The FAs know how to deal with feigned huffing and puffing