can work out (say a 3 mile bike ride or run, with some hills, at a fast and steady pace) without feeling out of breath, like you're going to die/have a heart attack, faint, etc. Also I believe you should consistently be able to do that like every 3 days, but also you need to vary it up too... you don't want to just be strong in one area... health is fitness but also your mind state as well... there is a lot to health..
i guess if theyre bodyweight is not too thin and not too big, altho iv heard that people on either side can also be healthy. that's not your question tho:P
also, if they look toned that's a healthy sign that they look after their muscle condition. complexion is one also because the amount of vegetables and calcium consumed makes a huge difference with that. those are the main ones I guess=]
Usually you can tell from their skin and the way their body shape is, if they are over weight then tey probably aren't healthy, if they are really skinny then they aren't healthy, also mental health is a factor as well
Looks don't say much, unless the person is so obese that they can't run. Or so skinny that you see every bone through their skin. Maybe red eyes, that could indicate stress or lack of sleep. Yellow or broken nails, they don't take care of themselves. Other than that, there isn't really much to go on. You'd have to look at their habits and behavior to determine their health.
I can look at a person and tell if they are unhealthy or not. Certain things I was trained to look at. Like if they have a goiter, complexion is weak, hair and nails is also weak. Men will have more fat on their abdominal area, women will have cellulite on their butt. Bruising on the legs. Body fat% to lean mass.
A person's BMI and by extension their weight is usually a good indicator of their overall health (not too skinny and not overweight). Other things that could add to a healthy body are a good posture (standing up tall, no rolled over shoulders or forward head posture, no anterior or posterior pelvic tilt etc.) All of these things add up to create a healthy looking body.