Some people come across so nice, proper, and put-together (nicely dressed, eloquant with speech) that you don't realize who they really are until you become really close to them and know them for years.

Are you able to spot insincerity in people who come across so nice and proper? If so, how? I know there is no one-size-fits-all rule to this. But I would like some examples of moments in which you just got that "hunch," or that "gut feeling," that someone is not being sincere.

P.S. The goal here is self-growth. I want to become more sharp and discerning as my counselor told me to explore "the young part" of myself that trusted so easily and in turn, got taken advantage of in the past.