It's easy to tell if your senior dog is in obvious pain: He's limping or whining, and reluctant to let you touch the area that hurts. Subtle pain is harder to distinguish. But you know your dog better than anyone else. If he displays behavioral or physical changes for no apparent reason, observe him closely. It might be time for a visit to the veterinarian.
Animals usually try to hide pain, since in the wild obvious infirmity makes them easy pickings for predators. Any sort of behavioral change could be a sign that your dog isn't feeling well. Maybe he's snapping at another dog or cat in the household whom he never minded before. If he always eagerly greeted you at the door upon your arrival but now doesn't bother to get up, something's amiss. Any sort of personality change is your dog's unconscious way of letting you know there's a problem.
Sooner or later, people and dogs usually experience the joint degeneration known as arthritis. If you catch the early signs of your dog's discomfort, your veterinarian can prescribe or recommend joint supplements to keep him active and pain-free longer. Even if your dog moves normally, suspect some aches if he no longer wants to climb up stairs or jump onto a favorite chair. He might sleep more than usual and appear grumpy or less inclined to participate in favorite activities.
Trust Your Gut
Your dog can't tell you that he's suffering some discomfort, opposed to outright pain. Since you know your dog so well, trust your instincts if you feel he just isn't himself. Write down your feelings and observations so you can relay them to your vet. For example, if he didn't finish his dinner with his usual gusto -- but he did eat it -- take note of the date and what you gave him.
Semiannual Veterinary Visits
Once your small or medium-size dog reaches the age of 10 or so, it's a good idea to take him for semiannual veterinary checkups instead of annual ones. If you have a large-breed dog, you might want to start seeing the vet regularly twice a year once he reaches his seventh birthday, since big dogs age faster. Your vet will check your dog's vital signs and give him an overall examination, and you can ask if certain subtle behaviors you've noticed are cause for concern.