Would you report someone if you thought they were abusing the system? If so how and to who?

Most of us know that one person, military or civilian, who seems to always be out of the office or always gets the new equipment or assignments that make you wonder. Of those there are some that you are 99% sure that they are abusing some system and not expending their leave or signing for equipment, etc. Would you report someone if you thought they were abusing the system? If so how and to who?

What are your thoughts?