My dreams are usually quite logical so it is hard to pinpoint a moment where something is odd or out of the ordinary. The way I got over this (not completely just yet) was just to work on my dream recall which has lead me to have more vivd dreams. By doing this I'm much more aware of things in my dreams and my dream surroundings, and can more easily spot the signs that indicate I'm dreaming.

In my dreams it's usually weird speech patters or words that tip me off that I'm dreaming.