How do most admins keep track of active computers in active directory

There has been a number of computers that are inactive.  We never had any sort of master list to keep track of active or inactive computers. Now we must do some housecleaning in our active directory and remove computers that no longer exists. What do most admins do or what is there approach for house cleaning the AD  environment? how would I know what machines are inactive so I can remove them from active directory