When you subconsciously think that every old man, court jester or storyteller in a book is Hoid. Seriously, I routinely think I spot Hoid in the books I read. He's everywhere.
When you see the words, "It's the journey, not the destination" in a book and nearly have a heart attack.
When random song lyrics remind you of Kaladin or other characters.
When all your friends, who haven't read the Cosmere, know certain release dates of upcoming books because you can't shut up about them.