If you hold the female in front of the male's cage so they can see each other, you can tell by her reactions if she is interested in mating or not. If she hisses, sways back and forth, darkens her background color, lunges in the direction of the male, etc. then she is not willing to mate. If she remains calm, keeps her body slim, walks slowly away from the male, etc. then she is ready to mate. Watch the male for aggressive behavior too though because sometimes they will react as though she should get out of his territory.
How old is she?