There are 7 body markers that indicate pain: 

• The head being positioned behind the vertical position, 10 degrees or more, for 10 seconds of more
• The front of the head being in front of the vertical position, 30 degrees or more, for 10 seconds or more
• The head repeatedly moving up and down, not in rhythm with the trot
• The head repeatedly moving from side to side
• The horse tilting its head on one side, with the ears unlevel
• The horse either clamping the tail to the buttocks, or holding the tail to one side, repeatedly
• Repeated tail swishing