Healthy dogs are active, especially when they're puppies. There should be no scabs, fleas, ticks, or hair loss. There is reason for concern if there is any discharge from eyes or nose (Although some dog breeds, like the Pug, do have some discharge naturally.)

If there is any coughing there should be concern. His ears should be clean and free from any black debris and not inflamed. Gums should be a nice pink color. Making noises is a good way to check for deafness. 

Always ask for a written guarantee from the breeder, rescue staff, and a vet. The guarantee should allow you to return the dog if the vet determines there is a serious problem or that would be costly to correct.

Dog health books can be found here