You would have to monitor everyone and look at the patterns of each Tech, watch for consistantcies and inconsistantcies. After a period of time, you will notice who is doing it right and who is not.
It would take time to establish a baseline of time it takes for PM's and repairs, if you run one product line this would make it easy, factoring in the customers location, the kind of people your are dealing with, do they drag their feet and make you wait to get at there equipment, does it take an hour to find someone to sign your paper work.
A good dispatcher(s) will be the key here, he should know where customers are at and how long it takes to get from place to place.