How can i tell if my dog is pregnant?

Take her to the vet about 4 1/2 weeks after she was mated and have an ultra sound scan done.Her nipples may be more prominent and pinker than usual. Depending on the size of the litter you may notice her losing her "waist" and becoming big tummied between 5 and 6 weeks after mating, though depending on her shape/breed it is possible she could be pregnant with a very small litter and you not notice til later still. Have you got the "Book of the Bitch" by Kay White and JM Evans? Its an excellent book to have handy.