I have already undergone the basic Mary Sue Litmus Test, and gained a lower score with Silver Star Apple than Princess Twilight. Or Season One Twilight. In any case, this one seems tailor-made to MLPFIM, so it should yield more precise results.
Let's begin.
( ) Your OC has more than 2 colors in his/her coat.
( ) Your OC has Bat Wings/Ears.
( ) They have an ethereal/flowing mane.