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.
Well, if it takes 65 days on average, this picture would've been taken on about day 50 or 51 (stray cat so I don't know for sure). If their belly looks like it could have a whole orange or softball inside it.. You're gonna have more cats soon!