They just know that one note doesn't sound right. If you are a beginner and mainly playing one note at a time, you may be able to go longer between tunings. Certain types of music show tuning problems more than others. Chopin with his large spread-out chords will show bad tuning much sooner than Bach who tends to use many melodies at the same time. In general twice a year seems to do for most home situations.
Some people go with once a year.
In school or church locations, because of the lack of humidity control twice a year may not be enough. Typically, the pianos are bad in August when school starts. By November the heat has come on and it is very dry and the pianos need tuning again for Christmas programs. If the piano holds up when they turn the heat down over vacation they make it until spring when the rains come and the weather gets humid again. Most institutional places don't tune three times a year but this is what they really need.