By people observing their neighbors are never there. In small communities everyone knows everyone, and it's in our own interest having a vibrant community, not lots of empty housing only being used during high season.

But mainly it's checked by seeing if any person has their primary residence registered at each address zoned to housing. Or by filling in a form saying it's rented out to someone living their temporarily, like seasonal workers, students at the ski academy etc.

But people do try to skimp around it by living there on paper while actually living somewhere else. But than at least part of their taxes paid goes to our county. County can force them to sell, though.