I would say I am very good at spotting insincerity - it is a learned behaviour.

I observe people constantly. Perhaps it is a sad thing but I make up opinions about people (IRL) from the get go. It comes from developed observations skills; and, the need to constantly be vigilant of what goes on around me. Namely, based on experience and traumas, I don't have a great deal of trust in people and thus need to make up my mind early on whether or not they pose a threat to me. I had to learn then from childhood how to spot such things.

I am not even sure actually of what tips me off. It need not be an eye contact thing as a narcissist will make very strong and forceful eye contact. Nope, it is other things I observe... how do they behave when they enter a room? What is their non verbal communication and body language? How comfortable are they in their 'persona'? etc.

All I know is i can spot something like insincerity from a mile away.