Feel free to add comments, suggestions, questions, and so on.
(Note: I use "she" throughout the test, but most of this can apply to characters of both sexes).
-Is the character's name your own, your nickname, your screenname, or a slight variant- for example, Kelli instead of Kelly? (10 points)
-Is the character's name a noun or verb not often used by other people in your world for that function? (4 points)
-Is the character's name completely inappropriate to the setting, such as a Japanese name in a world with no other trace of Japanese language or culture? (5 points).
-Is the character's main name a pseudonym? (2 points)
...that she chose as a reminder of her past? (Add 2).
...that is more "profound" or "cool" than her original name? (Add 1).
...that anyone with half a brain could use to connect the character with her original name, such as "Emilia" instead of "Emily?" (Add 4 points).
-Is the character named after a famous hero of the world or an animal? (2 points)
...And she has a mystical connection with that hero or that animal? (Add 4).
-Is the character's name a modern one in a medieval setting, such as Tiffany or Brittany? (3 points).
-Does the character have an eye color that is unusual/rare among her people or species? (1 point)
...that is unique among her people/species? (2 points).
...that is nonexistent in nature? (Silver eyes with blue lightning bolts, etc.) (4 points)
-Do the character's eyes get described more often than anyone else's? (2 points)
...long after the reader should know what she looks like? (3 points)
...with many adjectives that are close variations of each other? (Blue eyes that are also called azure, cerulean, 'like a summer sky,' cobalt, etc.) (4 points).
-Does the character have long, silky hair that flows down to the middle of her back or ankles? (2 points)
...that never gets dirty or tangled in branches? (Add 2 points)
...that she can leave free without braiding and never have to wash or tend? (Add 2 points).
...that remains free in the midst of battle? (Add 3 points).
-Does the hair have some unusual marking, like a white streak that does not follow a scar or multiple colors? (3 points).
-Does the character's hair get described more often than any other character's? (2 points)
...at inappropriate times, such as in the midst of battle? (Add 2 points).
-Does the character's appearance have a "reason" other than you just wanting her to look that way? (Blue eyes to prove that she's the reincarnation of a goddess, white streak to show that she's blessed, etc.) (4 points)
-Do other characters rhapsodize on about her beauty? (3 points) (Add 2 points if the character is one who wouldn't normally do this, like a taciturn soldier).
-Does she cause dramatic effects with her beauty, such as people fainting or traffic accidents? (5 points)
-Does she have scars that, no matter how ugly they would probably look in real life, just "add to her attractiveness?" (3 points)
-Does she have an unusual or symbolically significant marking or birthmark? (3 points).
-Does your character have:
...one dead parent? (1 point)
...two dead parents? (2 points)
...one or two murdered parents? (3 points) (Add 2 points if they were killed in front of her).
...dead parents who stayed alive long enough to make dramatic death speeches? (3 points) (Add 5 points if they were wounded in the heart or throat).
Is the character an orphan/foundling/of unknown descent? (2 points).
...who will dramatically rediscover her heritage? (Add 2 points).
...who was raised as a peasant but is really a descendant of royalty? (Add 7 points, and I would ask you to please reconsider. Everyone and their mother has done this plotline to death).
Was her family abusive? (3 points). (Add 2 points for every form of abuse on the list that your character has suffered).
-Being locked up.
-Being denied food or water.
-Family killed her pet/tore apart her stuffed animal.
Is the character the object of a prophecy? (10 points, and consider that note about reconsidering repeated here).
...whose coming has been delayed
Is the character running away from home? (4 points).
...despite knowing diddlysquat about surviving in the world outside? (Add 3 points).
Are the bad guys/good guys looking for the character? (3 points).
Skills and Special Abilities
Each skill from the following list is worth 1 point. If your character can perform the skill perfectly the first time, without any training whatsoever, then each is worth 3 points.
Destructive magic (flinging fireballs and the like).
Speaking to animals.
Speaking to gods.
Speaking more than two languages.
Mental powers (telepathy, telekinesis).
Is the character the "chosen" of a goddess/prophecy/elemental force/animal/big blue alien from Planet Yarkzarin? (5 points).
Does the character manage to do things that would be impossible for anyone else (such as dress as a man when she has big busty breasts, survive falls from tall buildings with all bones intact, walk on a broken leg?) (4 points).
Does the character perform magic that most people in your world cannot perform? (2 points).
Add one point for every condition that applies to your character's magic.
She can do magic normally reserved for:
People of higher levels of training.
The opposite gender.
Another type of mage.
People descended from a different bloodline.
Is your character THE most powerful/best mage or fighter in the world? (7 points).
Personality and Relationships
These are worth 2 points each.
Does your character have only flaws that:
Are not really flaws, like "too much compassion" or "too much wit?"
Do not inconvenience her, like not being able to cook but then never having to cook?
Are bad habits, such as biting her nails?
Can be airbrushed away at the end of the story, like timidity or arrogance that miraculously vanish?
Do your character's flaws and strengths contradict each other? (8 points). (This needs study. How can your character be both charming and socially inept, graceful and clumsy, arrogant and compassionate?)
Does your character instantly win the trust and affection of all the good characters? (4 points).
...alienate/make spiteful or jealous or fearful the bad ones? (Add 3 points).
...make other characters obsess over her even when she's not around? (Add 4 points).
...completely change another character's personality, such as redeeming a villain, or making the jealous bitch into her worshipper? (Add 5 points).
Do the character and her love interest fall in love...
...within a few months? (2 points).
...within a week? (4 points).
...within a day? (6 points).
...at first sight? (8 points).
Are they soulmates or reincarnated lovers who have been together before? (10 points).
Does the character cause out-of-character reactions in others, such as making the arrogant mage care more about her than his research, or impatient thieves put up with her whining about everything? (6 points).
Does the character never make:
Big mistakes? (2 points).
Small mistakes? (3 points).
Factual mistakes? (4 points)
Perceptual mistakes? (She knows who's good and who's evil, etc.) (5 points)
Are the character's perceptions identical with objective reality? (20 points). This is the biggest sign of a Canon Mary Sue.
Is there never any true competition for the main character? (5 points).
Is she the only one with any true personality? (7 points)
Female Characters Only
Is she a feminist in a world without a feminist movement or tradition? (5 points).
Is she persecuted by men just for being a woman? (3 points).
Is she a princess? (3 points).
...who doesn't want to be a princess? (Add 4 points).
...and runs away from home so that her parents will stop making her try to be a lady? (Add 9 points).
For Male Characters Only
Do all the women/men love him? (5 points)
Is he a maverick who goes against the authorities but never gets in trouble? (5 points).
Does he brood about things? (2 points)
...that are not his fault? (Add 4 points).
...and the majority of his life is consumed by his angst? (Add 9 points).
Your Relationship To The Character
Do you fantasize about being the character? (3 points)
...falling in love with/having sex with her? (4 points)
...falling in love with/having sex with her love interest? (5 points).
Do you act as the character outside the story (in a character journal, in an RPG, dressing up as her, etc.)? (8 points).
Do you think the character is above criticism? (8 points).
Does the character bear a marked physical resemblance to you? (4 points)
IS the character you? (15 points).
And tallying it up...
0-80: Excellent character-building. Go you forth and give the world a good fantasy character.
81-175: Well, she might have some Sueish traits, but as long as you don't get carried away, you've still done well.
176-250: Sue. Go after the points you've probably earned in the personality and skills sections, and try to make others' reactions to her and her own powers a little bit more realistic.
251-299: Um. You can pull this off if you're a really, really good writer. I would advise against it, though.
300+: Completely and utterly Sueish. She needs to be hacked and burned. Or at least get some of those ridiculous cliches chopped off.
That was fun.