PULL OUT YOUR driver’s license. Now look at your wedding album. Or the candid your best friend took of you busting up at the beach. Notice anything different? You should. The insincere smile, the kind we usually adopt for IDs, is physiologically different from the involuntary smile you get from a good joke or when you’re enjoying a date.

Back in 1862, in the book The Mechanism of Human Facial Expression, French neurologist Duchenne de Boulogne noted that an insincere smile uses only the muscles around the mouth, while a genuine smile involves everything from the lips to the cheeks and the eyes. "The first obeys the will," Duchenne wrote of the difference, "but the second is only put in play by the sweet emotions of the soul."

Duchenne’s theory got a boost in the 1970s and ‘80s by Paul Ekman, a psychologist at UC San Francisco who specializes in research on deception and facial expression. He even named the crinkling around the eye that occurs in an involuntary smile "Duchenne’s marker."

So how can you tell faux from real? Portland plastic surgeon David Magilke and Wanda Crook, a physical therapist who specializes in facial neuromuscular rehabilitation, agree that a true smile typically animates the entire face. "Smiles are more involved than people think," Magilke says. A sincere smile includes the ring of muscle that surrounds the eye, as well as muscles in the cheeks, nose, neck, and, of course, mouth. Look for action in the upper part of the face: the eyebrows commonly drop, crinkles form along the outer corners of the eyelids, the cheeks bunch, and the nostrils flare. Many of these movements are involuntary, and all are subtle, but they’re major markers of a bona fide smile. The insincere smile will almost always look simple by comparison.

Think you can spot a genuine grin? Take a look at these photos of prominent Portlanders and give it a shot. (Our experts’ best guesses are on the bottom of the page.)