I was also wondering if it would work that way in dreams or not. It wouldn't be hard to find out but it would probably be subjective depending on who did it. I don't expect it would make a very reliable reality check because of the time it takes tops to fall in real life, you would have to wait fo it to fall to know. Also I don't see why it wouldn't fall over in a dream unless you expected it not to. Last you would need a flat stable surface to test it on which might not be always an option in real life.