How do you tell the difference between shyness and something else?

Impairment. Most of us are shy in at least some settings. If you are shy but it doesn't interfere with how you live your life, don't worry about it. It shyness starts to rule--avoiding situations, refusing to go somewhere you really want to go, becoming a "homebody, " then it well may be more than just shyness and time to seek help with a professional.