I have a girl bearded dragon but it might not be a girl bearded dragon so how do I tell the difference?

A couple ways: First of all, males will have little bumps in the back leg area. Second, males will have wider heads than females. Then, most recognizable, is the EXTREMELY thick base of tail. It might even look ridicuously wide. These will only occur after "Hatchlinghood." Added: Head size is not always accurate. A healthy well fed dragon will have a thick tail base no matter what the sex is. You can look at the underside of the thighs. An adult male will have femoral pores which look like a straight row of bumps that may have a waxy look in the center. A female may have them but they are very small, pinpoint-like. You can also hold it in the palm of you hand, place your thumb on top and gently lift the tail upwards. Look at the base of the underside of the tail. Just below the vent (poop hole) on the tail side you will see either central mound (female) or 2 oblong bumps (male). If the dragon is an adult you should be able to tell pretty easily.
To see if your beardie is a boy or a girl, simply lay the beardie either in your hand or on a table facing away from you, gently lift the tail straight up and slightly over it's back, but be very gentle as beardies are actually pretty fragile, look just above it's "vent" on the base of the tail, if you see a hour glass shape between two larger bumps on either side of the tail, then its a boy, but if there is a lump right in the middle just above the vent, then it is a girl!

If you go to beardeddragon.org they have a section on sexing your beardie complete with pictures. Added; Finally a good answer!!!! I just wanted to add that if your dragon is still a juvie it may look like a female using this method. They usually start to show their sex by around 8-9 months. Also if it is smaller than normal it may be hard to tell. I ad a dragon that was a very sick rescue so was much smaller than normal. I thought he was a female until he was 2 years old! I noticed his large femoral pores on the inderside of his thighs.
When Beared Dragons are first born, the females and males look the same. But when they start to mature, you will start to notice that if your Bearded Dragon is female, its skin will start to add an orangey color.The male however, will not turn this orange color;it will stay the same color as when it was born. ANSWER If you hold up your dragon so you can see underneath it, Male bearded dragons will have glands at the top of its tail and under the rear legs. around there bottom ,if you prefer. These will look like little bumps. If you dragon shows no signs of these bumps around this area, it is a Female. It is inaccurate to sex them by their colour as colours are different for every bearded dragon. Hope this helps