Hi Liz, welcome to the Pregnant Cats and Kitten Care Forum.
It is extremely hard to tell in the early stages of pregnancy in cats whether or not they are. There really is no way to know for 100% certain without specific tests run by a veterinarian. That now being said, it is a very good bet that if you have an unaltered male running freely with an unaltered female you will eventually have kittens. *grin* Your female does not need to be in heat or even to have gone through her first heat to become pregnant. Cats are induced ovulators - this means that the act of mating itself causes the ovum to be released and if there is sperm in her reproductive tract, the odds are high in favor of successful fertilization. If your girl is young yet, having kittens to care for will be more difficult for her because she is still a kitten herself - she wants to have fun, to play and be a rough & tumble little girl, not be the good mother she needs to be.
You can usually plainly see a few symptoms at around 3 to 4 weeks into the pregnancy ... a more noticably rounded shape to the lower belly and abdomen, an increase in food intake, some girls even get a bit of morning sickness - and from my experience, this can be split 50/50, but some girls get really affectionate and loving while others get downright rude and nasty. At around 6 weeks into the pregnancy, you might also start seeing small, sudden movement when your girl is reclining. If you were to take a stethescope and listen you could even discern little heartbeats in there. At 7 to 8 weeks, your girl will be quite large and movement may be cumbersome for her, she may not want to jump up or down into or out of her normally favored places. She may begin looking for a suitable birthing nest to deliver her babies.
Are you absolutely certain that caring for a pregnant girl and her litter is something you are ready to take head on? There is a financial obligation as well as a time comittment involved. You have to understand that you can't just put some food down, scratch everyone on the head and leave to go out with your friends. You have to help your girl with her babies, teach them the social graces of sharing their lives with humans ... or else they won't be suitable pets when the time comes for someone to take them to their new forever homes. Are you ready and willing to give up a large part of your life to do this?
If your cat is in fact pregnant, you ~do~ have a choice not to allow the pregnancy to continue. You also have a choice to allow the pregnancy, raise the kittens until they are 12 weeks old, providing food, litter and vet care for them and then find good, responsible, caring homes for them. Just so you know and completely understand your choices.