I guess you just ask someone how happy they are in general.
Edit: I found the study (see my other response) and they used this:
To assess life satisfaction, respondents were asked, “All things considered, how satisfied are you with your life as a whole these days?” They were asked to indicate their satisfaction with life using an 11-point scale ranging from 0 (dissatisfied) to 10 (satisfied). This measure has been shown to have appropriate external validity and has been widely used in cross-cultural studies of life satisfaction