There was an episode of Batman, the Animated Series back in the early 90s where Batman was stuck in a dream due to some shenanigans by Scarecrow.
The viewer was unware that the entire episode was a dream.
As the episode came to a close, Batman states to Scarecrow that he knew he was stuck in a dream. When the Scarecrow asked how he figured it out, Batman stated that earlier in the eposode he caught a glimpse of a book (or was it a newspaper), and he saw that the letters were all jumbled. Since the part of the brain that dreams and reads are no connected, he deducted that he was in a dream. Hence, why he couldn't read the book.
I thought that was awesome and still remember it.
Sadly, I recall that episode when I'm having a decent dream and I see jumbled letters...then wake up.