The problem is, it's a personal thing and you can't ever "know" in others:

"Insincerity by virtue of pretending to have qualities or beliefs that you do not really have"

Notice the "beliefs that you do not really have." We can't truly know what other people's beliefs are. That's why it's so scary to think someone is being insincere, we can't know what's going on, if they're actually insincere and/or why. My T once said she felt compassion for me but I responded, "don't I get to say whether you're compassionate or not?" and she said, no, it was her feeling, she was the only judge of whether she felt compassionate. I think hypocrisy is the same thing; we can perceive and feel someone as not being sincere even if they are.