It generally works by asking people how happy they are + statistics to work out causes or correlations. I gather psych people do this kind of thing all the time about seemingly subjective things.

WP on measuring happiness: https://en.wikipedia.org/wiki/Happiness#Measures_of_happines...

More generally: Measures of emotion: A review <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2756702/>;