When doing your RCs, you want to be 50/50, or on the fence as to whether or not you believe you're dreaming. You don't want to make any assumptions until you do the RC and get a result from it.

That being said, it's more of the mindset done along with the RC that will make you lucid. People do occasionally become lucid from doing an RC, but they are usually aware enough in the first place and realize they are dreaming after doing the RC.