I hate Mary Sues, its true, but I also hate Mary Sue "tests", even if some of them are reliable. Just the idea that there's a "test," I guess, bugs me. The problem with Mary Sues is, pretty much *any* original character (especially one the same gender as the author) is GOING to be called a Mary Sue at some point. It's really an insult to a writer when people do that - I mean, original characters often stem from one's own personality and experiences, that's how original characters are *born*. So technically, there's a little of the writer in every one of their original characters (well, that's how I see it). And in a fanfic, there's usually a little bit of the writer in every one of the CANON characters as well, no matter how in-character they are portrayed. That's just the writing process. That's why co-authored fics are a million times awesomer than non-co-authored, in my opinion. That's why a show like Foster's has multiple writers, not just one. If the show had only one writer, it would be a disaster. You NEED multiple personalities and points of views to bring together a show, especially one like Foster's. Actually, when writing a fan-fic, I like to think of myself as sort of co-authoring with the writers of the original source material. That's the key I think - WORK with the existing material, don't reinvent it. When you reinvent it, that's what we like to call AU, or alternate-universe fanfic writing.

Anyways the same goes for original characters. If they honestly mesh with the existing ones, you're off to a good start. If they totally clash, *that's* a clue that you're heading in the wrong direction. Don't be afraid to ask others for opinions, but do NOT take the first person you ask's word for it. Because some people are a little TOO quick to cry "Mary Sue".
Just my opinion. And please don't take my Foster's fics Focus and Edge as examples; I swear I wasn't even trying when I wrote those. I was just bored, and it was before I was really and truly a fan of Foster's. That's why I stopped writing that series; as a FAN, I think it was a bad start.