It’s very hard to discern. Extremely.

Both narcissists and enlightened beings may appear narcissistic, and both will hurt you. The difference is that enlightened beings will hurt you skillfully, but at the time you will not know the difference - the pain will still be pain.

Here’s how you may be able to tell. If you’re coming to him at the time of need, hurting, vulnerable, and open to him about it (that’s the most important part, enlightened aren’t clairvoyant), and he “puts gasoline on the fire” to hurt you even more, chances are, you’re dealing with the narcissist. Almost all “enlightened egos” are still narcissistic, they just don’t know it. But if you’re coming to him with judgments, accusations, moral superiority, or any kind of superiority whatsoever, including superiority to other people, then he might hurt you, and that’s to be expected. Narcissists don’t like to be judged, also, but they love judging.

It will still be hard to discern, because the habit of attacking is so ingrained in ego’s patterns, that people use accusatory language at the first sign of pain as a defense mechanism. You might not even notice as you started doing it. You might actually be in pain already, so the response might feel uncalled for. But if you’re honest with yourself, it will make you think about the things you say and why you say them.

Online forums and all sorts of impersonal communication is by far the worst medium for any kind of conversations with an enlightened being, because they don’t transmit neither your intonation nor your intention.