Is your character a 'Mary Sue?' Find out! I found these quizzes online and while I wouldn't say they're 100% accurate, they'll definitely help you refine your characters. Most can be applied to both original & fanfic characters, although they are geared more toward fantasy/paranormal fiction. Some of the stuff is pretty obvious (is your character named after you, does he/she consider her exceptional beauty to be a curse, etc.), but other questions really surprised me. Who knew that being abused as a child contributed to Mary-Sue-ism? While I never really considered any of my characters to be 'Mary Sues,' these quizzes definitely made me take a harder look at them. I even made some changes to my MC based on some of these tests, and I think the whole story is better for it.
There are lots of other tests like this out there, but I like these the best because they don't require me to do any math. I click yes or no and it tells me at the end how saccharine and predictable my character is. Enjoy!