Very little is certain in our dreams as they are shaped by our doubts and expectations. If one doesn't expect that the light will turn on, it likely won't. If one truly believes the light will turn on, it probably will. And if one cares so little as to what happens then the subconscious mind will randomly choose whether the light turns on or not.

On this topic, there are no foolproof reality checks that I know of, only ones that seem reliable for most people.