We have a lot of parvo in my area, no one ever vaccinates and then the unvaccinated mother breeds and gives her babies no immunity and they break with it at the shelter.
The first sign is a listless puppy who doesn't eat and just doesn't look right. Then they move on to a bloody or white mucusy diarrhea. As a previous poster said, it smells terrible. Very coppery from the blood in it. As it progresses, most of them start vommitting.
Currently we are about 50/50 on saving parvo puppies. The most important thing is to keep them hydrated so that their little bodies can fight. They almost always need IV's, most of them can not or will not drink on their own. One of the worst things you can see is a puppy that hangs it's head over the water bowl but makes no effort to drink. That almost always spells fatality.
Any puppy with suspected parvo needs a vet immeadiatly, but call ahead. Don't bring the puppy through the front door, ask the staff where they want it brought in, many have seperate doors for contagious puppies like that.