First of all, you use your brain. Depending on your climate, a colony will need 40-80lb of honey to survive a winter. If they have less than that stored in the hive already, you will need to feed.
So, how do you know how much honey they have stored? You inspect, of course, and you carefully assess each frame of food stores. A Langstroth deep frame, when full, will hold between 4 and 6lb of honey, so let’s say 5lb average. So you need 8 to 16 deep frames completely filled with honey to get through the winter. If your bees have that, you may not need to feed, but my guess is that very few beekeepers leave that much in their hives. Also, if the winter is unseasonably cold, warm or long, you may still have to feed. The only way to be absolutely sure is to weigh the hives. I have a hive scale permanently installed under my hive, so I accurately know how much they have used (when it works! ).
I think Ed (@Red_Hot_Chilipepper) was describing an unusual situation where he took over some hives which had probably been fed excessively for a prolonged period of time. It took him several seasons to get rid of the stored syrup from the hive(s). It doesn’t mean that you shouldn’t feed, just that you should feed judiciously when needed, and stop when a flow is expected. I would rather feed and risk getting syrup in the honey, than not feed and lose a $150 - $200 colony of bees.