So essentially the customer wants to see if the server is idle or not? CPU util = 0, and memory usage, etc?
What does the customer consider "used" to mean?
I'd think you'd have to determine what services are installed on the box (database, webserver, appserver, etc) and then determine how to tell if that application is still being used (logs?)
I don't think that's the best way... it could not be idle because it's getting patches or people are logging in but it's essentially decommissioned.