Much of the training to become a doctor involves the physical exam. This is the time-honored skill of performing tasks like listening to the heart and lungs, palpating the abdomen, feeling for pulses, etc.
(Many people currently will also argue that the physical exam is obsolete, being rapidly replaced by lab tests, imaging, and other fancier technology. Personally, I don't entirely disagree.)
One of the most important parts of the exam, however, is the mere task of inspection, which involves just looking at the patient. This simple act can provide volumes of information. I feel a patient's hands, and in particular their fingernails, are very telling and provide useful information that the patient herself will not reveal. Here are a few findings I have observed over the last few years.
Dirty nails -- It doesn't take much time or effort at all to maintain fingernail hygiene, so if a patient has clean and trimmed nails it is inconclusive. However, nails that are ungroomed and dirty usually suggest clues about the patient's underlying social history, such as a broken or non-existent housing situation (e.g. as an extreme, being homeless).
Clubbing -- A classic, and the most medical of all these. Clubbing is the buildup of material at the proximal part of the nail bed, giving them a very exaggerated curvature. Often a sign of occult lung disease or cancer, if you notice clubbing consider evaluating the lungs in some detail. I once had an old man present with a set of very vague symptoms. When I noticed his hands and toes had severe clubbing, I started a workup that revealed metastatic prostate cancer.
Long fingernails -- Subtle, but in my view, very telling. I have noticed that long, unclipped fingernails often indicates someone with psychiatric issues. A mind fraught with psychosis or depression has more pressing issues than clean fingernails. Again, it doesn't take much to maintain one's fingernails, so when I see this I wonder if the patient is mentally sound.
One long, groomed pinky fingernail -- Historically this was a trait of gangster bosses, drug dealers, or the like, who proudly displayed a long and groomed nail to signal that they were white collar, and not a manual laborers who used their hands to make a living. Today, it means you're a douchebag. While this finding doesn't provide true medical information, it is always good to know when you're dealing with a douchebag.