How do you know if a female dog is pregnant?

Give her a Canine EPT test.

The ONLY sure way is to have the vet do an xray about a month before the pups would be due and 'see' if there are any puppies.

i have a female boxer that i got her a mate, that was around a month n a half ago. she after a month her nipples started to grow bigger than usuall, but at the same time sometimes it shows that she is fat from stomach but sometimes it doesnt. the part where it did increase was around her rib cage. is she pregnant/ or how could i know?

to know for sure you should have her xrayed.
but by the 40-45th day her nipples will increase in size and she will start poppping out on the sides and/or her stomach will appear lower... she may also start acting more like she is "uncomfortable" like stretching and rolling around

did you plan to mate your dog or was this an accident

i want to know

First she must have been bred during the time that conception can take place. If you believe she was, or you were trying to breed her. Then, about 28 days afterwards, you can take her to the vet and likely he/she can diagnose for you. Pregnancy usually lasts about 63 days, give or take.

If this was not a planned breeding I would strongly urge you to get this girl SPAYED AS SOON AS SHE IS FINISHED with her current heat cycle.

If you plan to breed her PLEASE ALSO PLAN to show her so that you earn your points and titles and can attract a dog for breeding consideration that also is worthy to IMPROVE the Golden Breed line you want to start.

Become a RESPONSIBLE breeder who CARES about weeding out the many problems that Puppy Mills and Back Yard breeders have helped to create in this WONDERFUL breed of dog!

Spaying will significantly REDUCE the likelihood that your Bitch will EVER get breast cancer, and it will REMOVE the chance to develop Cancer of the Uterus completely!