This photo of Prince William, Prince Charles and Kate Middleton (Catherine, Duchess of Cambridge) attending Christmas Day services at St. Mary Magdalene Church in Norfolk, Sandringham, shows both William and Kate with different mouth configurations, yet with similar eyes. The newly weds both are wearing suppressed smiles. Most people don't realize that a sincere, felt smile is not present unless a characteristic appearance of the eyes occur (Ekman). In order for a smile to be true, there must always be a dynamic and partial eyelid closure. And yet it is not this simple....
Some will tell you to look for "crows feet" when assessing "eye smiles" - but I caution against this for several reasons. After one reaches their mid-thirties, most everyone has some degree of crow's feet. Moreover an increase or decrease in these wrinkles are difficult to qualify, let alone quantify when the face is moving. A much more accurate and dynamic eye metric for identifying sincere smiles - is to look for eyelid closure. There is essentially no variance in iris size from person to person, so along with how much white of the eye (sclera) is visible - this adjacent anatomy serve as excellent yardsticks to quantify eyelid closure.
For a sincere smile though - simple partial eyelid closure is not enough nonverbal. There are different muscles used depending on the emotional reason for this action (See Secret # 353: Brad Pitt's Eyes - Sincere and Insincere Smiles). Sincere smiles must always have the characteristic concave-up furrow on the lower eyelids as well as their partial closure - as seen here very well on Prince William and to a somewhat lesser extent on Kate. Both are trying to suppress their sincere, felt smiles.