Look closely at the person standing by checking the way the lower and upper legs align, looking at the angles in the knee, muscle balance, ability to extend fully (hyper-extended?) and any more details.
Check see what the condition(s) & history are present regarding pain, movement limitation.
http://www.bartleby.com/107/291.html, Henry Gray (1825–1861). Anatomy of the Human Body. 1918.