HELP!!!! My dog sniffed some other dogs runny poop, does he have parvo?

Is your dog up todate with all his/her jabs? If not the dog shouldnt be anywhere apart from your backgarden. If your worried, take your dog to the vet to get him checked over.

we don't even know if the runny poop dog had parvo

If you're worried about it go and see a vet, you're unlikely to get a diagnosis over the internet

probably, go to the vet before it gets very sick