One of the main ways is that you come away from conversations and interactions feeling very confused.
They have a way of twisting everything. Manipulating facts, using your weaknesses. It's very unnerving.
if they do something wrong, they try and shift blame - at the same time they try and take credit for other people doing well.
They tend to show off a lot. And they try and impress people a lot.
if you catch them out, or back them into a corner, they can turn very, very, very nasty. They can also repaint history - to the point that arguing with them becomes mind bogglingly difficult because they literally do not acknowledge the facts as they actually happened.
I would say though...the main way is...you come away from interactions feeling confused as I said at the start.
A had a business partner who was a MAJOR full on narc, and he almost drove me to a breakdown because he manipulated me so heavily and throughout it all I did not even realise he was doing it.
For example; he would do something wrong - like take extra salary for himself, and if I tried to talk to him about it, he would:
a) Try an convince me I'd taken extra salary a few months ago and be so convincing I would actually start to question my own sanity.
b) Make me feel he deserved the extra salary because he was better / put more in.
c) Made me comes away from the altercation somehow feeling like I was the bad guy.
When you come face to face with someone like this it makes you feel very stressed and shaken inside because they are basically liars who will say or do absolutely anything to preserve their image (even in their own minds)