Narcissists can be very charming when you first meet them. Not always in a slimy flashy way - they can also play the self deprecating nice guy/girl, helpful, courteous, liberal etc. Watch out for someone mirroring you - throw a few wrong answers in there to test them out. We respond to people that we think are like us and who have similar values and the narcs know this.
Take your time in getting to know someone and don't be swayed by superficial impressions. Narcs are very good at ticking the boxes and impression management. They will hide behind a good job, relationship, groups of friends that they keep sweet, community work, religion etc. Pay attention to your gut - if it's not happy for some reason, even if everything looks good on the surface, then be very careful. Don't ignore WTF??? moments/comments. He might have aspergers, she might be very stressed or they just might be a narc whose mask slipped.
They present a false picture which is hard work and as they have no real empathy they are acting out emotions so they slip up sometimes. Notice emotional responses that feel fake. A bit too much?? They are acting out what they have seen others do and they can get it wrong. They can have different personalities for different situations. We all do this to a certain extent but there should be some continuity - not like an actor switching between characters.
They have a huge sense of entitlement to everything - you are merely a bit part actor in their drama - watch out for noses being put out of joint if you are successful at something. or have more of something - they won't be happy for you. It can play out in different ways - mean with their time but entitled to yours, expect attention but give none, their problems are important but not yours, mean with their money but entitled to yours, too grand to do menial work or bother with ordinary workers (the waitress test - how do they treat them?), entitled to the best even if on low income etc.
They do not take responsibility for any mistakes - they are always right. Therefore they don't grow as a person and will keep on making the same errors. They lack insight and will repeat patterns of behaviour even after pretending to change. They lie with ease and if caught out will switch lies in midstream or get defensive and angry to head you off if they can't think of something.
Basically IT'S ALL ABOUT THEM! If you have self esteem issues be careful as narcs are takers and those who feel unworthy for whatever reasons are givers. Look after yourself - be selfish - do your own thing - guard your own space - know your own mind. Learn about your vulnerabilities and hot buttons (guilt etc.) as narcs will play on them to get what they want. Make sure you have good boundaries.
There's loads of info online, books on amazon - read up on them so you have a better chance of spotting them and steering clear. Do your homework - they will be doing their homework on YOU after all!