You may have heard the joke: How to you tell if a lawyer is lying? Answer: When his mouth is moving. While there may be more than just a grain of truth in that joke, there is quite a bit of information which can be found by observing faces and not just those of lawyers.
If a person smiles and only one half of his mouth forms the smile, he or she could be suffering a stroke or perhaps a less serious condition such as Bells palsy. Bells palsy results from a paralyzed facial nerve.
If one pupil (the dark circle in the center of the eye) is not the same size as the other, this could be from something relatively benign such as using glaucoma drops on one eye or having a minor trauma involving one eye. On the other hand, brain tumors, strokes, glaucoma and a whole host of other nasty things could be the reason.
If a relatively younger person (to be politically correct, let us say under 50) has lost most of their teeth, he or she could have a serious disease such as AIDS, diabetes, or cancer, have used crank, or might have suffered trauma (think fist injury).
If there is a red rash over the central part of the face, this might be due to an autoimmune disease such as lupus, be the result of drinking a lot of alcohol, or even be from an allergy.
If the outer part of the eyelash is missing or thinning that person might have a thyroid condition. Graves disease which is caused by a high thyroid level can also result in bug eyes.
Thin lips with tight facial skin can be seen in Sjogrens syndrome, another autoimmune disease.
Facial twitching might occur with some inheirited neurological disorders, with certain types of seizures, with tetanus and with low calcium levels.
A bulbous deformity of the nose known as rhinophyma occurs with heavy alcohol use.
In fact, the list is much, much longer. What can you see in a face? In turns out, quite a bit.