It depends on the individual.
But imo a vibe of then never being there for you when you need them but whenever you're really busy with something important you can bet they'll expect you to drop it and support them in some way. The friendship you start to notice, isn't reciprocal.
They treat partners icily too. Or you start to realise their relationships always seem to involve drama. They may even brag about mistreating their partners.
You start to feel like they view your life as their competition. To notice they keep bringing every topic back to them.
They tell lies and exaggerate things right infront of you, to others and you think 'but that's not true/how it happened'. But at first you let it slide because its not a big deal...really.. is it? Until, it is. But even then, if you call them on it, YOU would look like the drama lama.
They may take an interest in the hobbies of you or other friends and make it 'their thing'. You then feel put off it. They may also steal your mannerisms, style, things you've said word for word...you think we'll, emulation is flattery. Until its not.
Other possibilities are bragging and boasting. Telling you how shit their life is constantly yet total disinterest in changing it...you realise they like being miserable.
And (common one) once you've spent time with them, you feel worse about yourself. You feel tired, sad, down about your life, have low self esteem suddenly.
They may ignore certain social boundaries. Eg: walk closer to you than others would. Bumping into you and making you think you're the clumsy one.
They may tell you other people don't like you or say or think certain things about you.
A good test - knock something over in public near them. Or fake fall/injure yourself. Do they hang back and help out? Or do they run away or look at you like you're a piece of shit? If it's the later two (and assuming this is someone who is meant to be your friend or partner) you might be dealing with a narcissist.