How Does a Dermatologist Check for Miniaturization?

They use an instrument called a dermoscope (trichoscope)! It looks at the hair with 50 times magnification.  This view shows the value of the instrument, especially when combined with the experience of the person using it.  Below is a picture of a patient with alopecia areata showing blunted exclamation mark hairs (black arrows) which makes the diagnosis for the expert evident. The red arrows show miniaturized hairs.