I think it's difficult to tell if oneself is thoughtful, but identifying other people as thoughtful or not is not nearly as difficult, if you have known them for any amount of time. If they are commenting on a topic you know about from personal experience, is their commentary more or less well informed than the average person?

We are certainly all fallible, but thoughtful would not imply infallible, only that you are thinking over (in this case) the actions of the person in this case, prior to deciding if they were wrong or right. A fanboy, or a hater, more or less by definition knows the answer ("right" and "wrong", respectively) before thinking about it much.

Paying attention to the opinions of other thoughtful people is a much better strategy than only paying attention to your own opinion, with all of its perspective problems. If all the thoughtful people you know have the opposite opinion of you about someone or something, it doesn't necessarily mean you're wrong, but it ought to at least prompt some double-checking on your part.