Open their eyelids - shine a light into their eyes - do their pupils contract? Caveat: there are some drugs and diseases that can produce a non-pupil response in a living person - so go to other tests - like a pulse along the carotid artery in the neck, put a mirror under their nose while holding their mouth shut - does it fog from exhalation? Have they gone pale? Is there any sign of lividity? Reflex responses? Swallowing reflex? Got a blood pressure cuff? O2 saturation finger monitor? Deep coma can even confuse doctors trained to look for signs of life - which is why you should not make this assessment unless you know what you're doing. Call the EMT's.