When her face isn't in sync with the rest of her body. Like, you can notice that her body's skin tone isn't as attractive / healthy as her face. Look at exposed arms, body, cleavage, back. Hard to put in words.
Some girls are very good at it that I simply can't tell (unless I get her in bed and observe her the next morning - all in the name of scientific research). Some I can tell it's makeup, but it's so well done that it doesn't matter.