Hasn't this ever happened to you? Picture it: you are with a bunch of friends, chatting and having a good time. Then, someone leaves, and people start to backstab him, criticize him and whine about how better off we are without him.

And when he/she was there, they were nice and talkative, laughed his/her jokes et al.

This makes me think if I would notice if people hated me. They'd probably act normal in front of me. Yes, like they do right now. And there's no way (no ethical way, at least) to check that out.