When performing an at-home physical on your pet, its important to look for subtle lameness. Subtle lameness could just be the result of a simple sprain or strain, or it could signify an early onset of arthritis, infection, or even a back problem.
It's sometimes difficult to determine in which leg a dog may be painful, especially if a front leg is involved. To pinpoint the area, observe your pet's head during movement. A pet with a sore front leg tends to land heavily on its good leg as it shifts its weight away from a painful one. In other words, a dog will lift its head when its lame lamb hits the ground in an attempt to ease or "lift" the weight off of the limb.
If a dog is experiencing pain in a rear leg, it will extend the neck and drop the head when the lame limb hits the ground (the hip on the painful side will also rise slightly). You can also test for pain in the limbs by having your dog stand upright on all fours. Proceed to lift each foot off the ground one by one. Dogs will resist someone lifting up a good foot/leg, since this means that more pressure must be placed on the bad leg. Conversely, when you lift the painful leg, little resistance is encountered and the dog will gladly shift its weight to the sound limb. Dogs experiencing lameness in both limbs may shift from side to side when standing still.
Yet a third way to tell if the limb is hurting is to look for muscle atrophy or wasting in the shoulder (front limb lameness) or thigh (hind limb lameness) regions. Compare both sides for symmetry. If you notice the muscles on one side seem "smaller" than their counterparts on the opposite side, it could mean that leg is hurting and that your dog isn't using it as much, hence the muscle atrophy on that side.
If you detect a subtle limp on your exam and it persists for more than two weeks or gets worse, have it checked out by your veterinarian in order to find out what's causing it. Depending on the cause, early treatment can often head off or slow the progression of serious challenges that could lead to expensive medical interventions on down the line.