Question on pregnancy

My roommate and I have been taking care of an outdoor neighbor cat for several months because their house burned down. We fell in love with her, and now they have offered her to us. We don't know much about her medical history. They said she has had three litters, and they sort of unconvincingly said she was spayed. Now, her belly seems to be getting a bit big, and being the queen of worrying, I am wondering if maybe she is pregnant. The reason I am afraid she is pregnant is because the belly is also kind of firm when you touch it. I don't know if there is some easy way to tell the difference between a cat who is getting chubby from eating a bit too much, or a cat who is expecting. Also, I know she has not gotten pregnant for the last year. It seems to me, the chances are she is not pregnant, but I wondered if anyone out there had some insight on the subject.
Thanks...

Like said above, checking the nipples and feeling for movement is all you can do at home.
I would set up a vet appt, just because you don't have a real medical history. If she is pregnant, you would want to make sure you are prepared.

Even vets can be wrong, so be careful!
One of my foster cats sometime last year was said to be pregnant (which is why I took her, prepairing for kittens and all.)
She had been to 2 vets, they both said "yup I think she's pregnant, probably due in 3 weeks."

4 weeks past, she had the appiete and the size of a preggo! But hadn't really gotten any bigger, and then my fiance called me complaining of all the noise this "darn cat" was making (I was away from home for the weekend at work.) and I had to say "uhhh I think she's in heat then!"

And I was right, took her back to the vet and he said "oh, it must have just been gas before, because she's defeintly not pregnant."

They do have ultra sounds, or sonograms? (which one?) for animals to tell if they are REALLY pregnant or not.