If she is bleeding, has a swollen vulva, or has a brownish or straw colored discharge.

For a detailed answer to that and just about anything else on the subject, refer to "The Book of the Bitch". If you closely observe her on a regular basis, you will notice a few pronounced changes. Normally, the female's plumbing will become swollen and tender. She will have a discharge that changes in color over several days and behavioral changes will be evident, such as a preferance for shorter playtimes and more frequent naps -- usually off in another room away from the rest of the family. Although still very sweet and lovable, there is a bit of resistance to being groomed and a picky appetite. When her season is over, you will be able to tell right away. She will pursue you to play more and she will eat more predictably. She will have the exuberance of a young puppy and everything will be just dandy with her!