Here's what I tend to take into account:
-Is the person self-assured (good posture, direct eye contact, relaxed expression, speaks without fumbling, not fidgeting) or do I detect shyness/anxiety?
-How invested is this person in appearances? Are they wearing the latest trends? Does it look like they've spent a lot of time doing their hair? Are their clothes revealing or modest? Are they designed to attract attention? Does this person seem to spend a lot of time and money on their appearance and looking nice/wealthy?
-When the person smiles, if he smiles, is it genuine? Is it a reflex? Is it a nervous appeal?
-How polite and gracious is the person? Does he have nice manners or is he abrupt and no-nonsense?
-What does the person talk about? The last football match or the book he's writing? Does he find ways to take the conversation to a deeper level or does it remain superficial? Does he show interest in the other person and try to draw them out? How does he respond to questions - with disinterested one-liners or with intriguing comments? Does he know how to ask ordinary questions in a fresh way?
I think I understand quite well who someone is at that given moment by looking at and briefly interacting with them. Some people are unpredictable and often have hidden layers you didn't expect. Others look interesting but are quite shallow, nothing more to know. Usually, I can sense whether this is a person who has a lot of substance or is superficial, at least at this moment in time.
'Admiration is a very shortlived passion, that immediately decays upon growing familiar with its object'.
(Addison, English novelist,1672-1719)