You have to use your own powers of observation. Watch for stiffness in the dog's gait. Watch for abnormal behavior, like licking or pawing a certain area of it's body repeatedly. Watch for head shaking. More than an occasional shake may indicate an ear problem.
Inactivity is a major indicator.
If your dog's activity level suddenly decreases, it could be indicating that he doesn't feel well. It's hard to know what could be the cause, but it would be a good idea to keep your eye on the dog's stool, checking for parasites or foreign objects. Go over his whole body feeling for any unusually warm areas, which might indicate a swelling or infection. Take a look at the anal sacs. Dogs which regularly require grooming services probably get their anal sacs expunged at the groomer's.