Probably the best thing is to measure the number of NEET people. People who are physically and mentally able, not otherwise occupied (eg. raising a family), and for no apparent reason choose not to work or study.
People generally like and need to be productive in some fashion and it’s good for society as well so this is a good metric. However, it should be noted that people are not expected to work throughout their lives. Breaks are expected and a healthy thing to do.