I think there are various types of astral entities, and the way to tell 'who' they are is an art in itself.
General guidelines: If they tell you something about yourself more than likely they're self aspects. If they reflect fears or doubts back at you they're probably self aspects. Many guides are self aspects. That doesn't mean they're 'your imagination', it means they're part of your multidimensional self (aka collective soul, or oversoul).
If they give you advice that seems dangerous, they could be external entities, but not necessarily. Never follow advice that seems wrong, because saying 'no' may be part of the lesson you must learn.
A way to tell if it's an external entity that does not wish you well, is how you feel after the experience- if you feel sick, drained, exhausted, it could have been a negative entity, and if so, psychic self defense countermeasures would be in order.
If you get 'verifiable' information, verify it. It doesn't mean that you 'have' to do something, sometimes the point is to 'get' that it's 'real'.
Cheers.
As for meaning, I'd say study symbolism (like Jungian archetypes). Or are you already doing it?