I, too, rescued a pregnant stray, and she was absolutely enormous before she gave birth. (about triple her regular, non-pregnant size she looked like a deflated balloon afterwards). If you can actually see kittens moving than she is likely close. I don't know how friendly your cat is, she may decide to hide when she gets closer to giving birth, or if she is really really people oriented, she may want company, it depends on the cat. A few days prior, she will start looking for nesting spots. the day of she may start pacing, and she probably won't eat much, if anything, the day she gives birth. Since up to that point she will likely have been gobbling everything in sight, this is an easy sign to look for, although every cat is different and has their own way of doing things.
Dr. Seuss~DLH (brother's cat)~June 2007-
Misty~DSH (my cat & Mooby's mom)-?-
MooBoots(Mooby) ~ DMH(Mom's cat)~July 21, 2008-
Sunshine~ Golden retriever X white german sheperd (Dad's dog)~October 24, 2008-