Earlier quoted context omitted.
Make it part of the onboarding process! Fresh eyes are critical in a way you haven't been in ages - it also doubles as a way to initiate new hires into your processes. If there's a problem - they might even notice it and learn something along the way!
This is a great idea, but unfortunately many runbooks can only be meaningfully evaluated in a situation in which they are useful. That is - you need to instigate an outage (ideally, in a controlled reversible way[0]) before you can "test" a runbook. [0] https://sre.google/
If you're following Google's SRE all of your infrastructure should be pretty easy to replicate with a sample of production data. You can try to crash it in calorimetry to your heart's content, away from production systems. You might even learn (or teach) some things through the process of creating and attempting to destroy a test bed of this nature. They (https://sre.google) cover this in chapter 17.