This is done by applying mental and physical stress...evaluate the dog's response and actions during the process...and then observative the dog's recovery time. A good dog should take these things in stride. Again...to really understand requires first hand witnessing and experience...see the post above because when I stated it was like writting about directions on how to ice skate, drive a race car, dance, sing, or make love...I meant it. Some things just can't be explained. Reading a dog is a large part of it btw, and something you can't just "write off."