Broken bones usually aren’t life-threatening, but they require immediate medical attention. Prior to bringing someone to the hospital for a suspected broken bone, there are some basic first-aid treatments that can help minimize pain and prevent further injury.
The most common symptom of a broken bone is intense pain that worsens when the injured person tries to move. If the injured area is numb, swollen, visibly deformed or bleeding heavily, that could also indicate a break.
See http://stevefarzam.net/first-aid-for-broken-bones/ to read about the initial steps you should take if you suspect someone has broken a bone.