The easiest way to see if you are dreaming or not is to try to flip on the lights of a room.
In dreams, there is never any variance in light, so if you tried to turn the lights on or off, nothing would happen.
If you try this during one of your dreams, don't be surprised if you end up waking up out of the excitement of discovering that you are dreaming.
If you are able to concentrate enough in a dream to try to turn a light on or off to see if you are dreaming, you are a very good candidate for lucid dreaming, which is something VERY fun!
If anyone is interested in knowing more about this, PM me.