I've seen people asked to leave who claimed they didn't notice, but I guess that if you argued there weren't any seats in standard then you've pretty much confessed.
I can't speak for all Southern trains, but I presume they're quite clearly marked as first class even if the 2+2 seating is the same as the rest of that carriage. On FCC, however, there were, and maybe still is, at least one 321 that for whatever reason (cleaning, repainting etc) have no stickers to show first class - and ONLY have the antimacassars on the seats. If you stepped in and someone was sitting on all the seats but one, which you sat down on without turning around, you could probably claim you didn't know - yet I doubt any RPI would accept it.
I can't speak for all Southern trains, but I presume they're quite clearly marked as first class even if the 2+2 seating is the same as the rest of that carriage. On FCC, however, there were, and maybe still is, at least one 321 that for whatever reason (cleaning, repainting etc) have no stickers to show first class - and ONLY have the antimacassars on the seats. If you stepped in and someone was sitting on all the seats but one, which you sat down on without turning around, you could probably claim you didn't know - yet I doubt any RPI would accept it.