A common sign of happiness is rapid hopping up and down, similar to popcorn popping. (hartz.com)
Yet there are individual differences in such facial displays of emotion, governed by variations in facial muscles or nerves; a smile may be dimpled or not while still remaining a recognizable sign of happiness. (scientificamerican.com)
(It's also okay if it doesn't have any obvious signs of happiness.) (elementsofcinema.com)