Paleontologists can tell the difference between herbivores, carnivores and omnivores by the type of teeth that they had. Herbivores have flat teeth, while carnivores had sharp teeth and omnivores had a combination of the two.
Hyenas are carnivores and scavengers. As a group, hyenas are well able to drive lions off a kill if they outnumber the lions. They will also drive off and take over a kill that is old enough to be classed a scavenging carrion. They have been documented as hunting and killing in their own right.