I've spent a lot of time researching and trying to practice lucid dreaming. There is no way to prove it's a dream when you're in it. I've heard various techniques such as spinning around and if you don't get dizzy - it's a dream, or look at a page of printed text, look away, then look back at it - if it's changed, you're in a dream. I've tried some of these techniques but I still don't think there is any way to prove you are in a dream.