you'll know when you see it
my female boxer had pups almost a year ago and it only took a few weeks and she started to look like a cow. Her teets got so big and her whole under side looked like it was sagging to the ground. It's really funny actually, it a cute way. You will probably want to keep her from a lot of commotion in the last few weeks of her pregnancy because she'll get fiesty like a pregnant woman ready to burst. When Jezzebelle was almost ready to pop, my roommates dog, Hampton, was so scared of her. He would hide because she wasn't up for any playing at all. She would fight him if he even tried. It got to the point that Hampton wouldn't even go in the same room as her. Also, once she has the puppies, you have to be really careful of who is around her when she is with the puppies. It has to be only those who she trust really well. They get very protective of the pups and aren't afraid to bite you if she thinks your trying to intrude. We had 3 people get bitten who we thought jezzy was cool with. The first day they were born was the only time she wasn't like that. I think it was because she was so exhausted from the delivery. It took her almost 12 hours to deliver 9 pups. It was her first litter and she was 6 years old so it may have been harder on her than usual.
Keep an eye on her and watch the calendar for her approximate due date. I hope you have a healthy, beautiful litter :)