Hello:
Has she ever bred before?
Do you have a stud on site?
How many days into the heat cycle is she?
Females are most fertile between days 9-14 of the heat cycle. You should give your vet a call and ask to bring her in to have the test done to see which stage of estrus she is in..an estrus test. There isn't anything over the counter, that I am aware of. Basically they take a sample from within the vulva via swab, and check it under a microscope. Each stage of estrus looks differently. You can ask your vet if you can swab her and that may save you an office call with your girl. My vet will allow me to pick up the swabs, swab my girl and return the swabs to him for checking. :) Good luck to you and hope you have a healthy happy litter. :)
breeder/healthcare specialist
15+ years animal health care Hobby breeder of Olde English Mastiffs and owned by an English Bulldog