I'm an artist disguised as a retailmanager
I have found once that something or an entity/guide was working through me with paintings, years ago when I was studying for my bachelor Art degree. I could feel the difference and that way I knew it must have been 'something else'. It was positive.
Otherwise I would say that you can probably learn to tell by exprience. I don't worry about it as I'm very imaginitive. I'm sure most of my wacky ideas and creations are my 'own'. It's very likely you can tell by the experience you have that it's not you 'making it up'. When you're not sure just leave it be. It doesn't always matter. Feel comfortable about what you do.