how do i know if my bitch is pregnant?

Only sure way is whn she gives birth:D Seriously though in canines this is not easy as most bitches will have some signs of phantom pregnancy, so will exhibit all the usual signs that point to pregnancy. that is nipples turning pinker, and glands becoming more defined, change in behaviour etc.

An experienced vet can feel during a short time around 3 to 4 weeks in whelp, but even then there is no certainty. Ultrasound is more reliable to confirm pregnancy, though unreliable regarding numbers. Again it depends on the skill of the vet doing the scan.

HI BEK
Another sign in my girls is the vulvae will still stay enlarged in size and will not return to its normal size, You may also see a jelly type discharge from her vulvae which is a very light color she may get a increase later in her appetite and as stated in other post nipples will enlarge slightly and go a deeper pink then normal some bitches may suffer from morning sickness I found my old Stafford did bother times she was in pup but would only bring up flem and later on she will start enlarging around the rib cage.
sandy.

I always palpalated mine at 4 weeks and was able to tell, but if u don't know what u are doing better to have a vet do it. U can usually feel how many there are to, but the further in pregnancy u can't since pups are way too big. Sorry if this was mentioned already.
ttfn :)

Another sure way to find out is to take the bitch to the vets for a scan, that way you can also get an idea of how many to expect.
I have never had my bitches scanned as I think the un necessary stress my do more harm than good. But if the bitch has had problems getting pregnant in the past a scan may be a good idea.