I have a feeling that doctors look at people's nails without telling them, because when nails are showing signs of illness it is usually pretty obvious, like if they are pale, or concave, for example. A doctor can probably spot these signs by seeing your nails as you sit there.
It's interesting, a few years ago when I went through a period of intense physical and psychological stress, a horizontal line appeared on my nails and eventually grew out. I think the body draws resources into where they are needed the most in times of crisis.