To me a Mary Sue is a character, that doesn't seem to be realistic. Like someone who is good at almost anything even when they do it the first time. Click the link below to see a definition. http://en.wikipedia.org/wiki/Mary_Sue
If you aren't sure about your character is a mary sue and you want to write a fanfic, just think about it. If you write in an "I" point of view, then I think your character is a mary sue, since you favor him/her more then the other characters. The main rule that I go by, is your characters shouldn't be more powerful then the main characters, like Artha and Beau (we wouldn't want ocs hogging the spotlight).