I hate to tell you this, if you do not know how to tell if your dog is in labor...what are you going to do if she has problems in whelp? I do not think you would know, dogs die in whelp when people do not know what they are doing.
Answer- she will go off of her food before she has the puppies. She will pace, dig, get up and down, start to pant. Sometimes their water will break before puppies start to come out. Some dogs shiver, and some dogs like to be left alone-they will hide. Some dogs will not leave your side and want to be with you. You can feel contractions- eventually you will see her start to push and eventually you will see a puppy come out if all goes well. I would recommend that you go online and read about the process of labor for a dog and what you need to know. Good luck-