Having followed your posts here and on #lesswrong, I got an impression of your personality as a bizarre mix of insecurities and narcissism (but without any malice), and this comment is no exception. You are certainly in need of a few sessions with a good therapist, but, judging by your past posts, you are not likely to actually go for it, so that's a catch 22. Alternatively, taking a Dale Carnegie course and actually taking its lessons to heart and putting an effort into it might be a good idea. Or a similar interpersonal relationship course you can find locally and afford.

Some things are easier to tell whether you're good at than others. I guess you aren't talking about the more assessable things (school/university studies, job, competitive sport, weightlifting, ...) but about things with a strong element of judgement (quality as a friend or lover, skill in painting, ...) or a lot of noise mixed with any signal there might be (stock-picking[1], running a successful startup company, ...). [1] Index funds are the canonical answer to that one, but you know that already. So, anyway, the answer to "how do I tell if I'm any good at X?" depends strongly on X. But maybe you really mean not "(know if I'm actually any good at) anything" but know if I'm actually (any good at anything)" -- i.e., the question isn't "am I any good at X?" but "is there anything I'm any good at?". The answer to that is almost certainly yes; if someone is seriously suggesting otherwise then they are almost certainly dishonest or stupid or malicious or some combination of those, and should be ignored unless they have actual power to harm you; if some bit of your brain is seriously suggesting otherwise then you should learn to ignore it. There are almost certainly specific X you have good evidence of being good at, which will imply a positive answer to "is there anything I'm good at?". Pick a few, inspect them as closely as you feel you have to to be sure you aren't fooling yourself, and remember the answer. If someone else is declaring publicly that you are a pretentious idiot and full of shit, it is likely that what's going on is not at all that they're trying to make an objective assessment of your capabilities or character, but that they are engaged in some sort of fight over status or influence or something, and are saying whatever seems like it may do damage. I expect you have good reasons for getting into that sort of fight, so I'll just say: bear in mind when you do that this is a thing that happens, and that such comments are usually not useful feedback f