for the doctor to feel your uterus abdominally ie. by pressing on your tummy, you need to be about 8 weeks pregnant, although that will be less for ladies who have been pregnant before.
the uterus then moves up about 1 cm for every extra week pregnant you are.
they may be able to tell earlier by doing a 'bimanual' examination, which is an internal examination whilst also pressing on the abdomen, as the uterus gets pushed up towards the abdomen.