I have a birth mark on my thumb and pinky finger on my left hand, so if I look at my hand in a dream would they be there? Would this work?
I wouldn’t rely on it 100%. The brain can be very sneaky sometimes so I would have two or three RCs if I were you, just to make sure. Pinching your nose and trying to breathe is a good one. It’s the only one that has worked well for me at least. This one seems like your brain could leave your birthmarks in the dream as that’s what you’d expect to see. You’ll just have to try it out and see if it works. 
Counting my fingers is very reliable, did it in about 50 LD’s and there was only one where it was negative (five fingers) . Not even worth the effort to do two or three RC’s to get that 2% extra chance to end up lucid in my opinion
Good RCs are all about what your SC would expect and what would it be in real life.
If you manage to trick your SC into expect something unreal, anything could work as an RC.
If your SC expects your hand to have no birthmarks, why wouldn’t it work as an RC?
It probably would, but remember, don’t use only one RC and don’t do RCs before pondering about their dreaming outcome, otherwise you might get tricked into FLDs or mistake it as real life.
For most people, I think it’s reliable because they’re told their hands are going to be messed up. 90% of the time my hands look almost identical to real life.
That’s why I always go for more than one. Last time I counted my fingers in an LD I had five and they were fine, but I was already lucid so kept checking. They were long and wobbley after that. Then I had fingernails growing down my finger! My point is you never know how your brain is going to react and a second RC isn’t much effort at all…