unless you are psychic and a doctor, no, you can't. it's terribly unhealthy to weigh 120lbs and eat nothing but lettuce and smoke two packs a day, but noone comments on the skinny ones and says "eww, they are gonna die of a heart attack" (which is just as likely for a 'skinny' person given the right genetics, habits, and conditions as it is for a regular slightly-above-average individual!)