Here is a great example of a sincere smile. Notice how Isabelle Giordano's upper and lower eye lids are partially closed. This is the most essential component of true, felt happiness/joy. So often we concentrate on just the mouth, both when we make a smile and when we measure the sincerity of others. Crow's feet may appear or be accentuated (if pre-existing / age-related) as well, but the partial eye closure is the key to a true smile.
Notice also how Isabelle's mouth only shows her upper teeth. The "squared-off" false smile which people will make, pulls their mouths more outward as well as pulling the lower lip down. You should see no more than a fraction of the lower teeth - any more and it's not a true, felt smile.
Also very important is the relaxed appearance of Ms. Giordano's forehead. Smiles where you see wrinkling of the forehead muscles - is an example of another very common sign of a false smile. Interestingly, this also tends to be used more in insincere personality types. Negotiators beware!