The same way that anyone would: they conduct a statistical sample and report unemployment numbers based on that. Source: 15 years ago I was one of the people they contacted. As I recall, a letter came to our apartment, asking for the adult (18+) who had their birthday closest to that date to contact them and provide a phone number and some basic demographic data. That was me, not my roommate, so for the next 12 months I got a call once a month asking me for my employment details about a specific week that month- had I been looking for a job, was I still employed, etc.