Anorexics don’t look like thin people. Or I should say, naturally thin people do not resemble anorexics–healthy people have a glow to their skin, their hair is springy and growing, they (usually) have their teeth. They are not cachexic, gray with small open sores dotted over their skins–anorexics and cancer pts are. In fact, to put it in the crudest of terms, they usually look like shit. There was a girl in my German class in college who was anorexic. I was at that time 5’7" and weighed about 112 (ah, youth). She was my height and easily weighed 30 or so pounds less than I did. She looked like a death’s head. It was painful to look at her. I was quite slim, but nothing like her.
I may be crazy, but after 20 years of adult acute care nursing, I can tell at a glance if someone is ill (with what I do not know) and thin or just thin. (the “normal” looking thin person could still be ill–a bulemic in early stages or similar), but the OP did not describe that.