Any mechanism of injury that can cause a fracture can cause an open fracture.
Besides the usual neurovascular exam that is indicated for all injured patients, examination of any wounds is key. Examine the wounds for visible bone. If the base of the wound cannot be visualized, the wound can be gently probed to help determine if there is communication with bone.
For obvious open fractures, it is important to determine the extent of soft tissue injury or loss. This is a factor in determining antibiotic coverage. Note if there is any visible contamination of the wound(s).