You can either have her blood tested which will tell you when she is ovulating, but this sometimes has to be done a few times. Or you can introduce your male and go by his reactions roughly around day 10 onwards, though some males will mate a bitch whether she is ready or not. I think your best option would be the blood tests. Speak to your vet about this soon so he is prepared for it.
A good book to buy is "The Book of The bitch" by J.M Evans & Kay White, you can get it on Amazon.

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