When you're holding Christmas, have someone hold her and put one hand over her face to kind of "hide" her. Then, gently [don't want to break the tail] pull her tail up about 90* and look at the base of the tail on the actual tail. If there's one bump, that means female and if there's two bumps [one on either side of the tail] that means male. Males and females will both turn black or dark to absorb more heat/light and if they're "bearding" they might black beard to be more intimidating.

Here's a good link with pictures on what to look for while sexing Christmas. Keep in mind though, if your beardie isn't fully grown, chances are it'll just look female. Beardies tend to look female until they're fully grown unless you have a vet probe them and even then the vet usually won't 100% guarantee it.