How can u tell when a female dog is pregnant?

I HAVE A HOTDOG AND SHE JUST TURNED ONE MARCH 26TH, AND JUST CAME OFF HER FRIST HEAT CYCLE IN THE BEG. ON THIS MONTH BUT NOW HER ARE GETTING BIGGER AND HER WAIST IS JUST A LITTLE BIGGER ALSO, I WATCHED HER WHILE SHE WAS IN HEAT, BUT IM WORRIED IF SHE SLIPED BY ME AND POSS GOT PREG. WHAT ARE THE SIGNS TO LOOK FOR ?