She starts eating a lot and the sides of her stomach get visible bigger. Not so normal signs, she'll protest and/or try to bite you if you pet her stomach, she might start acting strange (you know, hormones and stuff) and be cranky most of the time of sulk around. Basically a difference in behavior. After she's a bit along, I don't think you could not notice that she's pregnant. Cats are pregnant for about nine weeks. They can have 1 to 8 kittens at a time.
You can discern a cat's pregnancy by its physical changes like the swell of its nipples, increase in appetite, possible vomiting and enlargement of the abdomen. It may also begin nesting. It's gestation period is an average of 63 to 67 days and so it can be overdue. For more detail, visit http://cats.about.com/od/reproduction/a/pregnant_signs.htm.