Lameness  can be attributed to an orthopedic or neurologic pathology. It is important to ALWAYS  perform both exams.

To correctly assess a patient with orthopedic disease, the best way is to evaluate him or her when there is not excitement or stress. The first approach to a proper physical examination is to observe the dog from distance. No palpation should be instituted at this time. Look at the patient when is standing, walking and trotting. 

During standing look for muscle asymmetry, foot placement (partial or non weight bearing), and body weight shifting 

During walking and trotting, the patient should be observed on a short leash walk and  on a straight line.  Look at the patient from the back, front and side. The stride is also very important in dictating which limb is affected. The affected limb has commonly a short stride. 

If the lameness is involving the front limbs, look at the head position during weight bearing. For the sound limb (good one) the head goes down, vice versa for the affected limb the head goes up.

If the lameness involves the hind limbs the hip of the affected leg will be higher because of the weight shifting. Additionally muscle atrophy can be appreciated on the affected limb

The next step is  to palpate the patient. Feel for muscle atrophy, bony prominence, swelling, decreased range of motion and crepitus. The best indicator to detect pathology is pain response. 

To assess lameness attributed to a neurologic problem,  observe the patient for pain, foot dragging, knuckling, proprioceptive deficits, reflex abnormalities and nail wearing.

Do not be afraid to perform a lameness exam, it is easy, however it is important to be methodic.