Found pregnant Chihuahua: when will she give birth?

Hi. A little more than a week ago, my family and I found a Chihuahua running around the streets. We took her in, noticing she was visibly pregnant. When we were sure that no one had lost her, we started preparing a whelping box. She doesn't take much interest in it, but when we wake up in the morning, it is shaped like she manipulated it. Milk can be expressed, and you can see the puppies moving around in her stomach. The vet is bringing in an ultrasound machine soon, but she might have her puppies before then. Her temperature has been at 99-100 degrees for the past several days. She's clingy, too, and has had a clear discharge. How do I know when she is going to give birth soon? Any ideas or guesses? Thank you.