The tummy gets fatter. The babies can be seen or felt moving around. The nipples turn bright pink. Within 60 days of conception, they fluff up with little pillows of milk. Kittens are born.
You can't tell right away. After a couple weeks you may notice a slightly larger tummy and increased appetite. They also may seem moodier than normal. The last week or so she may start looking around for a nice 'cozy' place to have her kittens (usually a closet or an open dresser drawer - somewhere dark & quiet) You can try setting up a box with towels for her but if it's not just right for her, she won't take it