The easiest way is to tell if a victim is unconscious or not breathing is to shout at them! If the victim is unconscious or cannot breathe, the victim will not answer.
To deal with a victim who is unconscious or not breathing, remember your ABC's.
Airway—Check that the airway passage is clear of obstructions.
Breathe—If a person is not breathing, assist immediately. Once the airway passage is clear, listen and feel to see if the victim is breathing. Wait 5 seconds. If there is no breathing, breathe into the victim. This expands the victim's lungs and gives oxygen. Don't stop breathing into the victim until qualified medical help arrives!
Circulation—Check to see if the victim has circulation (a heart beat). If the heart stops beating, breathing also stops. CPR (cardiopulmonary resuscitation) is required when circulation stops. CPR combines mouth-to-mouth breathing and chest compression to get the heart and lungs working together.