If you notice a dream sign or something strange, and then u wonder, "am i dreaming?" then you are!! When I'm dreaming it hardly occurs to me whether i am dreaming or not, but if i simply think the question then i know i am. If it occurs to me whether I'm dreaming in real life then i instantly realize that I'm not. once i feel something is fishy i would try to change the dream, that seems like a good reality check too because if it works you're dreaming. Maybe this is just me though.