Not very well, as it turns out. Pain is very subjective. Although doctors, psychiatrists, and psychologists have been studying pain for centuries, establishing a good method to determine how much pain a person is feeling has been an elusive goal.
There are numerous factors that influence a patient's response to pain, beyond the feeling of the pain itself. A child, for example, may be more inclined to under-report their pain if they are questioned by a stranger. People may have varying levels of pain tolerance depending on their culture or personal experiences, and may therefore report the pain they are suffering differently. Even a person's emotional state can affect the pain they feel.
One of the most straightforward ways to assess pain is to ask the patient how they would rate their suffering on a scale of zero to ten, where zero is no pain, and ten is the worst pain imaginable. More sophisticated assessments, such as the McGill Pain Questionnaire, consist of batteries of questions that delve into how a person feels about their pain and how it affects their daily lives and relationships.
In the cases of young children, the mentally handicapped, and others who cannot verbally describe their pain, doctors sometimes use graphs displaying faces that represent various degrees of distress. The patient is asked to pick which face most closely resembles the way that the pain makes them feel.
Measuring pain in infants, however, can be a particular challenge. Some tests have been developed to assess infant pain based on facial expressions, voluntary and involuntary movements of the limbs, heartbeat, and breathing patterns.