Well, of course the most obvious difference is that a real dream has to include a message. Every culture distinguishes prophetic dreams as something separate, and for good reason, because that sort of dream comes from the outside, while the others only come from within.

[She hums softly, nudging his foot right back; she's still a little miffed about the lack of cuddles, but having a project to think about is certainly helping somewhat with distracting her from it.]

Another strong signal is being able to recognize that a part of the dream does not make sense. If it is a dream entirely of your own creation, then it will always seem to make sense, even where it is nonsensical. To be able to recognize that something is wrong is a hallmark of something else causing it.