I am trying to learn more about shoes, but for the near term goal, I have this query - how do I really know that I need an insole? How to select the right insole that I need?
You don't. You should ask to a orthopedic doctor about it (possibly one specialized in sport), and have proper measurement taken. Choosing a bad insole that goes against your leg building might seriously damage your joints and knees. Don't have a shopkeeper decide either, they are NOT qualified to know which of your leg is shorter or longer, how your knee and ankles turn, and if you need a specific lift for one or both of your feet.