1. A Mary Sue is described as being overly perfect. Every feature of her appearance is beautiful, and nothing about her is unappealing or even plain. Sometimes she considers herself to be ordinary or plain, but the other characters' opinions of her disagree with this.
2. Her physical appearance is often described in far more detail than any of the other characters. Sometimes an entire paragraph, or even multiple paragraphs, is used to describe her. She is often also described in over-the-top, flowery and romanticised language. Her features can be compared to gold, gemstones and other valuables.
3. Similarly, she has little or no character flaws and weaknesses. Her only issues are small things like being too nice, or clumsy, or having a little bit of a temper. These issues will never have a lasting negative effect.
4. She is amazingly skilled at a range of different tasks and rarely is weak in anything. If she does lack a talent in something, it'll be something irrelevant. She can solve any problem the characters in the story run into.
5. If the canon characters possess any magical skills or powers, it is likely that a Mary Sue is particularly skilled or talented in magic. She might have more powers than is normal in the fandom, or have a rare or even unique power. If prophecies exist in the fandom, there is likely to be at least one prophecy involving her.
6. Her past might include a dramatic tragedy, over which she initially focuses but eventually heroically overcomes. At other times, her past might be very similar to that of a canon character. Her future, on the other hand, is almost always a perfectly happy ending.
7. She steals the limelight from the canon characters. She might use a canon character's best lines or be given that character's heroic moment. The canon characters might think about Mary Sue or be fearful for her when in the original story they would instead be worried about another canon character. Some canon characters might be killed off, vilified or just omitted from the story in order to make room for her.
8. She is a love interest for at least one of the main canon characters, possibly more depending on the author's intent. She could also befriend or be a long lost relative for another canon character whom the author likes but isn't romantically interested in.
9. Her name is likely to be something flowery, beautiful or exotic sounding. It might be spelled unusually. She could also have multiple names. Alternatively, her name could be the same as the author's, the author's middle name or nickname.
10. She is likely to be based on the author in some ways. This could include sharing the same interests and opinions, being an idealised version of the author in personality and appearance, coming from the same hometown or having the author's dream career. As a result, the author is unwilling to criticise her or have anything negative happen to her.