Here are some things I keep in mind when I find it hard to know what is imagination vs what is 'real' in a vision, visualisation, meditation, or astral journey. I find it a lot easier to accept what is coming through for others, but not so much for myself. I also just think that the more you do it, the more you'll realise how much you can pick up intuitively.
Unless your visions are making you paranoid or causing you to take dangerous actions, there is minimal harm in finding meaning in what you see whether or not you know it's imagination
Imagination is one layer of this sort of message receiving. It is a vehicle for messages to be communicated, not an enemy. Such as how some people are very receptive to songs and may hear songs as a way of receiving - the songs are a vehicle rather than random
Write down your visions after they happen. Don't think about interpreting them until you come back to them later. Aka separate the perceiving of visions vs the interpretation of visions. Then you can notice patterns, symbolism etc.
I do tend to ask myself "is there a reason I would just start thinking of this?" or - does it feel like it's coming out of nowhere? Then it might be something
Sometimes, the imagery isn't as important as the "message". The feeling, the meaning, the interpretation, the connection. So sometimes you can put the visual aside if the message is clear, even of the imagery isn't
Not all visuals will be precise or meaningful even if your visual ability is strong. It may be fragmented, it may be abstract. That is normal as well