Diahrea (bloody especially, but bloody diahrea should be a red flag anyway you look at it), vomitting, and lethargy. Parvo kills the lining in the stomach which is where the blood comes from and why they vomit.
Also Parvo has a very distinct smell to it, very sickening in the feces form.
Also parvo lives in the ground for 6 months, not 7 years. It's still an incredibly long time though. And Parvo IS treatable, it's just not directly curable. But you can treat it, else it would have a 100% mortality rate, which it doesn't.
But I've never understood why people outright refuse to believe parvo is a problem. Every now and then we get people that deny the parvo vaccine because they think we're trying to milk them of money or something. Ludicrous. And no convincing, short of actually showing them a parvo puppy (if we happen to have any in at the moment) will change their minds. I feel for those dogs.
And what sucks is when they come in a few weeks/months later with, guess what?