How can you tell if a dog is pregnant?

My dog's nipples have gotten a little bit larger, and she seems to be sleeping and laying down a lot. Another thing, my husband feeds her and our other dog once a day, a cup and a half at night. Most of the time, she will eat half of it and I guess save it for a "snack" in the middle of the night and it will all be gone by morning time. Well, recently, she has been eating it all at night before she goes to sleep. We don't want puppies at all. Recently, a lady moved in with us and brought her dog. They have a big fenced in area that our dogs run around in. The deal was, she kept her dog somewhere else because both of our dogs are girls and we didn't want any puppies. Our other dog is a Shetland Sheepdog and def don't want muts from a 1000 dollar dog. Her dog got in our dog's fence, and when we put him back, he gets back in. So its very possible. He sniffs her pee everytime she pees. So please no rude comments on "well you should have gotten your dog spayed or neautured." Because it wasn't our decision, we keep our dogs fenced in.