Honestly i hated the term, because a lot of people throw it out there for any character with superpowers that isn't an original loved chararcter like Superman or spiderman. I've also seen it happen when the character has power's or crap in a universe/world where other people have it, in which case the character isn't really a sue. Anyways NANOWRIMO gave me this awesoem link today to test and see how much of a Mary-Sue characters are [link] I'm glad to say that i think i answered it right and the one character i tested scored only 10, so for now the characters not a sue, but i'm thinking about looking at other characters, and somehow i think they might be sues or close to them. depends on what context i use. like i originally wrote one version of bloodtypes and then i threw that into aback story that i've never submitted and if i were to go including that as existent then Vrittney scores a heck of a lot higher, anyway i thought the test was fun maybe someone else will too. oh biggest no-no on the test the character shouldn't be a reflection of you or what you want, nor should you get angery at the test, justing being angery gives you 10 points. though i have to say its for good reason. i've seen a few real mary-sues in rp's and its hard to miss the fact even thoguh i hate the term its the only thing that fits to discribe them and its usually acompanied with writers who get really tempramental about their characters.
I'm also going to go with this advice, if you do fan-fics you're probably going to crash and burn in this test, if you say its you're own story you migth do a bit better. anyway tons of fun time to read 2 chapters of physics