How can you tell when your older dog is in pain?

My border collie, Mikey, is 13 years old. he is a trooper and protective companion for my wife. compared to a humans life, Mikey is 91 years old. i know he must have arthritis. he sometimes yelps when he moves a certain way. the vet has given us arthritis medication for him. my question is this: should i continually give Mikey his medicene every 6 hours or should we wait to hear his next yelp? our vet was not much help. he left this decision to us.