How can you tell if someone is lieing by body language?

My sister is a really good liar, she doesn't do the stuttering or looking away from someone's eyes or any of that, but she has one telltale sign, whenever she lies, her nostrils flare. Which, if you don't know her or know that quirk, isn't a big deal, but I know it, and so it's very hard for her to lie to me.