MOI (Mechanism of Injury). This is a medical term for "how did it happen?". Some of this is common sense, if the victim's helmet is cracked wide open you should take C-Spine precautions. The same can be said for a victim that is unconscious (other than drowning), has spinal deformities, or complains about severe head or neck pains. A head blow with vomiting or unequal pupils is most likely a concussion. Ask the victim to squeeze both hands, if one is substantially weaker than the other you have the potential for spinal injury.
I hope you never encounter this sort of injury as we are typically unable to properly care for the victim. In general, we are left with some tough choices. Triage is the first order of business, can they breath? If not, carefully rotate the head and body so it is face up paying special attention to maintaining spine and head alignment. Assign one person to the head only! This person is the commander and all others take instructions from him. Move the victim as little as possible to get them to safety. EMTs use backboards for a very good reason, it is very difficult to safely move persons with C-Spine injuries. Begin your examination and complete a SOAP form if you have one in your first aid kit. Make a copy and send a pair of runners to get professional assistance whenever possible. Do your best to immobilize the patient and make comfortable.
Damage to the top of the spine (C1 & C2) can often lead to death. The rest of the spinal column, especially the upper part can lead to paralysis: C-Spine Morbidity. This is why we take our time in these rescues and leave extensive movement to the professionals.