Also check out the Base Report Pack > Infrastructure Changes template. You will see Adds, Deletes, and Modifys for whatever time period you specify. If you look at object type Virtual Machine, then In Modifys, power state of a VM shows up as a modifcation. So if a VM shows up in the modify list there's a 99.99% chance it was used that period.
As Vitaly pointed out, you use those templates you see to create reports you like and then save them with your own Report names.