Welcome to the site, and I'm glad you came here because this is the best place on the web to get answers (in my opinion
First, do not put a flea collar on her or use flea treatments if you think she might be pregnant. It's just like a human woman taking medicine when she thinks she might be pregnant. It's a no-no. The chemicals in flea collars and flea treatments soak into the skin and while it's normally not dangerous for an adult cat, it can be VERY dangerous to any unborn kittens.
You've been given very good advice on how to tell if a cat is pregnant but with some it's hard to tell until they're about ready to pop. If your kitty has been in heat (as in crying, calling or meowing for no apparent reason, acting strange or displaying her butt in the air, tail to one side, or VERY loving to you, always wanting attention and pettings) and suddenly stopped, she's probably mated and you're going to be a grandma. Also, check the nipples and watch her tummy. I took in a stray one time that I didn't know was pregnant and she ended up looking like a walking foot ball! Best bet is to take her to the vet and make sure. If she IS pregnant you should switch her food to kitten food because that has higher nutrients and she will need that during her pregnancy and afterwards while she's nursing. The cost isn't that much different.
If you already have homes for them lined up, that's good (keep in mind litters can be large). But please, after this litter, have your girl spayed. If you can't afford it at the vet, call your humane society and see if there are any lost cost spay/neuter clinics in the area or see if you could qualify for a voucher to help you pay for it.
You have to understand....a lot of people here work or have contact with animal rescues and shelters that get tens of thousands of unwanted kittens all the time and most never find homes and get put to sleep. Bringing new kittens into the world is actually cruel to them if they can't have forever homes.
Keep us updated on how she's doing!