Hullin 53: Clawed and Kosher - Maybe

When a kosher animal is clawed by a non-kosher animal - when do we take it as a given that it will be considered a treyfa -- or really to the contrary? What if the animal is a cat? Or a fox? Or a hawk? Why is the question what animal did the clawing, as compared to asking what damage has been caused? Plus, determining what happened to the clawed animal, when nothing was really seen. Where the clawing animal's claws poisonous? If so, doesn't that harm lead to the animal becoming a treyfa? The sages are establishing rules of thumb so that they can make assumptions about the clawed animal, rather then investigating every case.