It is difficult to understand if there is something wrong with your puppy. Sometimes the symptoms are not visible and since dogs do not speak human it make it even harder to tell if they are hurting.
If you are attentive enough there are signs that help you recognize if your dog needs to be seen by a vet.
#1 Unexplained weight loss . If they lost their appetite it might be because of a metabolic disorder, cancer, heart disease etc.