Like everyone said, it is by feel. It helps to ride as diverse terrain as possible for your region and you get the feel for where to be for what type of hill or continuous climb or false flat or headwind you are pedaling. You are riding well if you are shifting and asking questions about shifting. Like others said, it is an oh oh moment when you are in the lowest cog possible and there is still a lot of extremely steep grade stretching out before you, and so you want to bike enough terrain to figure out what makes an oh oh moment for you because that is when it is critical. The rest of it, you can shift up and down based on your cadence and it only takes a few rides before it is all automatic for you.