A person's words can always be manipulated. So can their reactions. But you can usually find a behaviour padron that reveals details of the personality, and personality is usually fixed.
1st glance analysis:
Upon meeting a person for the first time I go by certain guidelines.
1st: Vibe.
The vibe isn't something that can be explained. It's the feeling you get from the person. The malice or the ease. Empathy related, the vibe graps your instinct. It can show itself in the tone of voice, in a look that slips your face. It is not something you can rely on, but gives you a pretty good idea of the basic reactions of the person. Of course people don't always view a social relationship by their first glance, but how they react inicially tells you about them more than you could get from a conversation. You can denote intimidation, which relates to matters of sense of self and self esteem, agressiveness, related to fear usually, and other factors.
I usually like people whom I get an ease vibe from. Means they're not judgemental. And don't care. Are open to what is coming from you.
People I don't get a vibe from also fit my goos scenario, because if they don't care, I'm less likely to have to put up with testing and other social interactions I don't like.
Unease, whatever the reason, pushes me away, because I feel it, and it makes me too umcomfortable and distrustfull to pursue the relationship. Aggressiveness also pushes me away, because it makes me feel you have something to prove.
2nd: Look for more than meets the eye.
Don't see a person by what comes out of their mouth, but by the why it's coming out of their mouths. The subjects a person talks about, or the way the text is verbalized, can tell you the intentions of a person in saying those particular words. People usually tell stories that differentiate them, relate them or better them. Most people try to pass an image along with those words. Catch the image they try to pass. Sometimes it can be the opposite to what they really are like, which lets you know theirs weaknesses. Other times it can be the truth, which lets you know their strengths. The way of telling, tells you how a person sees others and the world, and herself, what she values most. Then you can relate or not.
I value goodness and fun. If you tell me a story in a fun way about something that happened to you, I'm bound to like you. If the story was bad, but you tell it in a fun way, I'll like you even more. Stories that make me realize you're altruistical and good minded are a plus.
If after my analysis I don't think I will relate to you in any way, I'll let you be, but I won't get close, because I conclude it wouldn't be a relationship that would bring me any return. Whatever you could have to teach me or to tell me, wouldn't be something I'd be interested in, so I back off and leave you to someone that would: ''Not all paths are for every walker''.
2nd phase: a deeper analysis
Once you have picked on the image the person is trying to pass along, the second step is too check it for consistency and truthfullness. a) You could have gotten it wrong. b) The package might not match the content. Both of these scenarios may make you see a person in a better way, or a worse one.
You keep the image of the person in your mind and then:
3rd: Look for what is there that shouldn't.
You can tell a lot from the consistency of the image by comparing the way people treat others in front of them, and behind their back. If a person talks to another nicely, and when the person is not there shows them discontempt. Shows flaws in character. If a person claims to consider another one dirt, and in front of them, is friendly. Shows envy.
In the opposite scenario a person that passes along an image of superiority that confronted with a daily situation acts in an altruistic way. Or a person you don't like at first glance haves an attitude you approve of that didn't match your first image.
Body language also plays a major role in this topic. The small signs we let escape, unwillingly in a moment of lapse. Neglect, or taking something for granted, may cause people to loosen their guard and reveal details we were unaware of. Like before... Those details can be good or bad.
4rd: Look for what isn't there.
If a person comes off appearing caring and doesn't show signs of it. If she tells you she likes you and their actions don't match. The lack of behaviour supporting the immage is a sign that you probably didn't get it right, and should focuse on the things you didn't saw before.
Well, if after all of this, you're person still matches your padrons, congratulations, you've met someone who probably will have a good effect in your life. If not, then you have two choices: use their weaknesses to keep that person around just in case you need one of their strengths, or let it go. I usually let people that I don't relate with go without much care towards it. To me it's as natural as breathing prefering relationships that are beneficial for both parts, over the ones who are so-so.
The people that fit my padrons usually get to stay in my life. (Unless our life approaches are too different - I can admire a person and still not want them in my life, because our life views would crash with each other). Sometimes for a short time, others for a longer. You can learn a lot from people who match you in certain ways, but who obviously have different ways in other fields. The view is never the same, and neither is the personality. And in relating to a person in these conditions, you can evolve yourself, and your point of views, learning from the best that person has to offer, and showing them the best you have to offer. The return goes both ways.