What does it mean that schemas can be inspected?

You can examine your own mental models and evaluate whether they serve you.

Example: You notice that every time a junior engineer proposes an unconventional architecture, your first reaction is skepticism. You pause and ask: what schema is driving that? You write it down: 'Unconventional approaches are risky and usually wrong.' Now you can evaluate it. Is that true? Sometimes. Is it always true? No — some of the best systems you've used started as unconventional proposals. The schema was invisible until you inspected it. Now it's an object you can refine.

Try this: Pick one recurring decision you make — how you prioritize your morning tasks, how you evaluate whether a meeting is worth attending, or how you decide which emails to answer first. Write down the rule you're actually following (not the one you think you should follow). Then ask three questions: Where did this rule come from? When does it work well? When does it fail? You've just performed a schema inspection. The written artifact is your evidence.