There are several ways that you can prove that an owner knew that his or her dog was vicious. In many states, you won't need to do this, but there's a chance the owner could claim that you caused the dog to attack.
One of the things you can use to show that the dog had violent tendencies is to look at the breed. Bigger animals tend to cause more serious injuries simply due to their size. Some breeds, like pit bulls or Rottweilers, have a propensity for viciousness that courts will consider.
Another thing to find out is what the animal's purpose was. It might be unlikely for a service animal to attack, whereas it would be more likely for an animal trained to fight or protect its owner to lash out. If the animal is normally locked up or in a cage, muzzled or has been in fights with other animals, those are all signs that the dog could have been dangerous in the past and that the owner should have known that you might be attacked.
If the owner warned you about the dog, that can also be used as evidence that the dog was potentially dangerous. This is something to keep in mind particularly if you saw signs about the dog being on the owner's property.
It's not your job to prove that an animal was vicious, and you should be entitled to compensation by law if you were attacked. Dogs' owners are responsible for the behaviors of their pets, regardless of the cause in many cases.
Source: FindLaw, "How to Prove Fault in States Without Dog Bite Laws," accessed Dec. 29, 2016