See- check if anyone saw what happened, stop the game and observe the injury.
Ask - question about how the injury happened?
Look - for obvious signs of injury e.g bruising, swelling, foreign objects, broken skin
Touch - the injury to identify painful regions
Active movement - can the person move with the injury?
Passive movement - if there is active movement gently move the injured area through it's full range of motion.
strength/stand up - can the person stand up?