Generally, you look at the molars for your first clue as to diet. Hippopotamuses* will be classified as herbivores.

*Don't put a Latin ending on a Greek word to form the plural.

Let's say at some future time, when hippopotami are extinct, and some future paleontologists discover some skeletons of them. Once they look at their skulls and specifically their teeth, are these paleontologists likely to think these are herbivores or carnivores? They have tusks that look like sabre teeth on cats, they have stabbing bottom teeth. Everything would tell me that this is a lethal predator, not some herbivore. What clues are there that this is a herbivore, given only a skeleton to study?

Check out this picture of a hippo skull below:

http://cdn01.wallconvert.com/_media/wp_400x250/1/2/12460.jpg