Westeros doesn't really have what modern science would consider meteorologists, so it's kind of hard to know exactly how these guys figure out when winter is coming. Granted, they live on a different planet with a pretty messed up seasonal system, but they also haven't discovered the combustion engine. Dudes haven't even mapped out the entire world, much less their weather system. That said, there is ONE way to know when winter is always coming...

game of thrones