I don't know about you guys, but 90% of the time I can tell if someone has spotted me or not using it when I'm coming from behind them. Most people (90%) try to look way too innocent or do stuff that gives away their intentions of counter-ambushing you. If they haven't seen you, they'll be moving in a way/doing stuff that they wouldn't do if they knew someone was about to stab them in the back. Also, keep in mind that if they are using the 360 view, their character will ONLY be staring forward, not turning even a bit.