If she is over 4 months old, not spayed, and has been outside unattended or had a tomcat inside with her, then it's a fair bet she's pregnant. Cats are extremely fertile.
If she's less than 3 weeks along, there's no way to tell. After 3 weeks, her nipples will start to get larger and pinker, and eventually the fur around the nipples will fall out. And her stomach will start to get larger.