Honestly I can't beleave you can't tell them apart. What kind of parent are you?
Just kidding.
I've heard of the nail polish thing. Also I've heard of taking a Sharpie and marking the inside of the ear. They come in different colors now so if you need to you can mark a couple of different ones. My two girls weighed exactly the same when they were born and have gained the exact amount of weight so far. So I have no clue who is who, but they are both doing well.