I think the gyno can tell by the actual test if you are preggo but they can tell if they do an internal examination they can feel if the uterus is big and at that point they may request you to take a blood test. Hope that helps

I also have heard that your cervix changes color when you are pregnant. It is a bluish purple tint, for some women. Not sure about that though. :)

Yes they can tell because the neck of the uterus turns a bluish purple color and is called the chadwicks sign.