This question is one I have often thought hard and deep about, and it caught my eye..
We can never be very certain of our first impressions. It is not that they are not always correct or not, either; Sometimes we misinterpret our own feelings and lead ourselves astray.
And there is another side to the question I almost never see someone ask or point out: When we ask, we tend to focus only on the bad or negative impressions. Why? It is just as equally possible that we could get a positive or favorable first impression about someone which turns out to be false. How many times have you felt someone was really good, or trustable, or otherwise a great person, only to find out they're "just like everyone else?"
That leads me to wonder.. Just how much of our first impressions really are a "gut instinct," and how much of it is ourselves telling ourselves what we want to think?
A common example I get from talking to many girls (I seem to give off the impression to girls that I'm the kind of guy they can talk to about their life stories :-P), is one where they met the perfect man, got excited that the guy took interest in them and asked them out on a date, only to have the date totally bomb out--Either they guy was too stuck on himself, or was checking out all the other girls, etc.
And all too often, these same girls tell me that "in retrospect, [they] should have listened to [their] gut instinct," which somehow was telling them not to go, but they didn't listen. When I hear that line, my first thought is.. Did you really feel that way, or are you just making yourself think that *now*, in hindsight?
Another common scenario I have heard is when someone gets a negative first impression, only to later find out it was false. Again, since girls talk to me a lot, I hear a lot of stories about how some guy really "creeped [them] out," but after getting to know they guy, they found out he was really a nice guy.. "just misunderstood."
There even seems to be a strong romantic appeal to being "just misunderstood," but it makes me wonder.. On the one hand, they're saying their first instinct, their first judgment about a person was too harsh, not to be trusted.. But now their opinions and views should be trusted without question?
I bring that last point up because it brings me to what I believe to be the crux of the matter. Most people make snap judgments based upon limited information, but few people want to even consider if they could possibly be wrong, especially if it casts themselves in a bad light. And who wants to think of themselves as being that "creepy, bad person?"
All of us struggle through our daily lives, fighting to believe in ourselves, dealing with issues of low self-esteem/image/worth/etc. We tend to build our own worlds, where everything is understood, makes sense, and could not possibly be wrong.. or if we're forced to accept that something we believe in is wrong, then we have some justification for our own error.
And there is nothing wrong with that--It's nearly impossible to live through life if you never believe or trust in yourself, at best. At worst, you'll only create a situation where your self-esteem/image is so messed up, you won't even know fantasy from reality, nor will you ever be able to truly trust another if you cannot trust yourself.
Putting it all in context of the question asked.. You're in a position now where you know you have to trust yourself on some level; But to be fair to others, you should also be wary and question yourself. With those two competing interests, how do you resolve it?
What I have found, that works for me (your mileage may vary), is that I pay heed to my gut instincts, as well as my head and heart. I take it all into consideration.. but withhold judgment until such time as I can verify it all. In the meantime, I can proceed cautiously, wary of any potential dangers or problems, and simply avoid them, without necessarily avoiding the person.
In short, I am not going to put myself at risk, but I am not going to assume or pass any judgment, either. I'm a mature, rational adult capable of observing, evaluating and thinking clearly. I value that above all else, even when I almost always trust my instincts.. they're rarely wrong. I learned how to truly read my own instincts, without letting my head and heart get in the way. But I don't discount them, either--They serve a purpose, as well. My heart guides me, and my head takes it all in, considers, weighs, and finally makes a decision. (I hope I'm being clear, even amid all the contradictions and competing interests! ;-))
The only exception would be when it comes to children. We teach them that if a situation FEELS wrong, it probably is, and to get away from it. For children, we should teach them that, until they have enough experience in life that they can start observing, recognizing danger signs, and making sound judgments.
I've only scratched the surface on my thoughts on this topic.. But this is already overly long. :-P