Dr. Dawson: How the deuce did you know I was a doctor?
Basil: A surgeon to be exact. Just returned from military duty in Afghanistan. Am I right?
Dr. Dawson: Why, ha, ha, yes. Major David Q. Dawson. But how could you possibly...?
Basil: Quite simple, really. You've sewn your torn cuff together with a Lambert stich, which, of course, only a surgeon uses. And the thread is a unique form of cat-gut, easily distinguished by its peculiar pungency, found only in the Afghan provinces.
Dr. Dawson: Amazing!
Basil: Actually, it's elementary, my dear Dawson.