Well several things are done, visual examination will show if the bone is obviously broken. X-rays will also show fractures in bones. If an x-ray unit is not available, lets say at a crash scene in the mountains, old school doctors would use a tuning fork to check for fractures in extremities. The tuning fork would be struck and held against the ends of the bones at elbows and joints. The vibrations travel through the bone, if a break is in a specific bone, the crack will vibrate causing a painful sensation indicating a break or fracture.