Ask yourself what you’d teach a younger kid for free on a Saturday. Whatever comes to mind is something you really know.

What you would explain for nothing on a day off is a good measure of what you genuinely have. Real knowledge wants to be handed on, and it does not need a fee to get moving. The subject that comes to mind first is usually one you underrate, because knowing it well has made it seem obvious.

Try it for real rather than as a thought experiment. Help a younger kid with it one afternoon, no payment, which also shows you where your understanding has holes. Teaching shows you that you knew more than you thought and less than you claimed.