DO NOT TRY THIS!
unless you have been properly trained
I would like to preface this with a quick statement: Never, ever, EVER try to clear or move a C-spine on the street. Call 911 and wait for the ambulances. This is a technique solely for use in the backcountry, where ambulances don't go and you want to move someone yourself after making sure it's safe.
First, make sure the patient has no immediate life threats. That is, make sure there's no shock, airway blockage, broken femur, uncontrolled bleeding.
Next, make sure the patient has had no drugs or alcohol in the last 24 hours, as these can obscure the signs you're looking for.
Finally, make sure it's been at least 3 hours since the suspected injury. Some symptoms of a C-spine injury may not manifest themselves for up to three hours.
If the head is in flexion: pull traction on the head and return it to a neutral postion. Never, EVER put someone's head in flexion if there is any chance of a C-spine injury - you will kill them.
First, palpate up and down the neck. If there is any point tenderness, STOP! The victim is unsafe to move. Place them in a recovery position and call for help.
Next, ask if there is any tingling or numbness in the extremities. See if the extremities respond to pain. If there is any tingling, or any loss of sensation, STOP! The victim is unsafe to move. Place them in a recovery position and call for help.
Next, pull traction on the head and turn the head 90 degrees left and right. If there is any tingling at any point, stop.
Next, stabilize the head and allow the victim to sit upright. DO NOT ALLOW THE HEAD TO GO INTO FLEXION! Pull traction and rotate the head back. If there is any tingling or numbness, stop.
Next, put an axial force on the neck - push straight down on the head. If there is any numbness, stop.
Finally, pull traction on the head and slowly put the head in flexion. If there is any tingling, stop.
If you reach this point, the C-spine is clear, and the victim is safe to move. Improvise a cervical collar from the victim's clothing or sleeping pad and carry the victim out. Be careful not to allow the head to go into flexion, as there may be an injury that you missed. All this exam does is ensure that the victim is relatively safe to move.