Jake had come in with a pronounced limp and Mr. Hudson had done exactly what any concerned pet owner would do and tried to find the sore spot. Some variation of this scenario plays out every day in the average small animal practice, sometimes several times a day.
I knelt down and greeted Jake, a friendly collie / lab / shepherd mix, and gave him a couple of his favorite liver treats. He wagged his tail and was going to try to lick my face, but I moved his head aside to begin my exam before he could. I don't mind the occasional dog "kiss", but I knew that Jake was also a notorious poop eater. I began to palpate and manipulate each limb from the toes to the top, starting with the apparently normal ones and finishing with his right hind leg, the one he was limping on. (Incidentally, everyone thinks there is something stuck in the paw, but that is very rarely the case unless you see the dog chewing at the paw.)
"Like I said doc, I already did that and I couldn't find anything that hurt."
Jake didn't react for me either, but I did feel a subtle swelling in his right knee joint and he had what we call a "positive drawer sign", in which the tibia (shin bone) is able to slide forward relative to the femur (thigh bone), a bit like a drawer opening slightly. This meant that Jake had torn his cranial cruciate ligament, called the anterior cruciate ligament, or "ACL", in humans. So, isn't that painful? If so, why wasn't Jake reacting? Yes, it is painful and Jake was not reacting because he is a stoic.
Not that many dogs or cats have human "ouch, that spot hurts" reactions to pain. Some do, but most don't. Most are either stoics or Cassandras. The stoics, like Jake, prefer not to show any sign of pain. This is in part because in nature showing pain can make you an easy target. This is especially true of prey species such as rabbits who are ultra-stoic, but it is also true of social predators, such as dogs, who might be in danger of losing status. That said, there is tremendous individual variation and tremendous breed variation.
So, if stoics won't let you identify the location of the injury because they won't show pain, what do Cassandras do? In their most extreme form Cassandras scream if you take a small step in their general direction. If they do let you examine them, they will show what seems like pain (more screaming) even if you are only vaguely in the vicinity the problem area. You might be able to generally localize the problem as "front end versus back end", maybe, but that's not all that helpful.
As a sweeping generalization, dogs are more likely to be stoics and cats are more likely to be Cassandras, but there is a lot of cross-over.
What is the poor veterinarian to do with the patient that not only refuses to speak English but is likely a stoic or a Cassandra? As Jake's story illustrated, we perform a specialized kind of physical exam where we feel for what might be swollen, out of place, loose and yes, in some cases sore. Sometimes xrays are needed. And sometimes even then we have to make educated guesses. Thank goodness for education!