Two important pieces of evidence for Homo erectus being a hunting species are: superior tool-making; evidence of systematic, rather than random, hunting parties.
Evidence suggests that Homo erectus was a better tool-maker than previous species. Fragments and whole tools found in and near ancient sites show a better grasp of symmetry, chipping techniques, and of experimentation to discover better methods. These tools are strongly linked to the Acheulean period, with the hand-axe being very strongly associated with Homo erectus. These tools would not be as useful if they were not used for specific purposes; hunting as a deliberate strategy instead of a random occurance required better knives and spears.
This moves into systematic hunting rather than random encounters. There is strong evidence that Homo erectus depended on native elephants for most of its food, hide, and fats. This provides the link between hunting and hunting in deliberate groups; since a single hunter would have trouble bringing down an elephant with stone tools, groups would need to mobilize in an organized manner to hunt efficiently. There is also a dig site in Olorgesailie, southwest Kenya, where a great number of stone tools were found in a pile of fossilized baboon skeletons; the stones were not native to the area, proving that Homo erectus had both organized the hunt deliberately and prepared beforehand with the proper tools and weapons.