For those who don't know what a Mary Sue is: see here.
The problem is, there are far too many ways a character can be a Mary Sue, as seen here. Many websites that offer tests to see if your character is a Mary Sue give pages of questions as well.
Is there a quick way to know I have created a Mary Sue?
[Please note: the above links go to the website TVTropes, which is an addictive time-suck. Visit at your own risk. —ed.]
If your character sounds too good to be true, then it is probably a Mary Sue. If your character always seems to be in the middle of everything and is always the one to resolve any issues/problems, then it could be a Mary Sue. If your character is more attractive/powerful/impressive than any other character by a mile, then it is probably a Mary Sue.
Unless your story is specifically about super-heroes, your character should not exhibit supernatural tendencies. Also, if you find yourself relating very closely to the character and feel that you are modeling it after your own traits/preferences/ideals, then you are definitely leaning towards a Mary Sue!
Basically, just evaluate whether or not your character is realistic. If it seems to be instilled with any form of talent that gives it an unfair advantage over other characters, then you need to tone it down.
One more important note: simply being aware of this pitfall goes a long, long way towards avoiding it in your own writing. Once you're sufficiently aware of the effect, with any luck you'll know it when you see it.
Obvious Mary Sues are easy to spot... subtle ones less so. Just as it's very hard to proofread your own work, it's often hard for us to see when we've made a character unrealistic.
My suggestion: get an editor. Or at least a beta reader. Find someone who will be honest with you.
"One day, nothing significant happened. Everyone got on as normal". What does your character do on that day? If they cannot get through a normal day, doing normal things - which may vary depending on the setting - then they may be a Mary Sue (a term I had never heard before, but I recognise the trope).
If you are worried that you are creating a Mary Sue, try writing a normal day for them, and see if it works. You will probably not have to go far into the day to know. @StevenDrennons answer and @Standbacks comment are an excellent checklist, but for a quick way to know, this might work. In fact, just trying to start grasping what a normal day would be might work.
