Okay, I had the idea a wee while ago and I'm not sure if it's original but I'll still pitch it to you guys.
Very simple. Get a friend to place a random object in a box. You must not know what the object is. Now, in your dream open the box and see if the object inside the box in your dream matches the box in real life.
I'm going to try it as soon as possible. Could be very interesting. If you try post your results here.