How to tell if the check engine light has been disabled in a Trailblazer

In my 12 years of owning my Trailblazer I never needed to know how the check engine light worked. While checking compatibility between 2003 and 2005 clusters, I found that the check engine light is turned on by the pcm. Now that I know how it works, and how it can be disabled, I'm sharing this so you know how to see for yourself if the light has been disabled. This applies for 2002 - 2005, I can't say that this is how the light works in the later years, but the tests would be the same.
While it might seem like I'm picking on Craig, and his precious list, this is really for any kind of (free) marketplace that people use to take advantage of unsuspecting people by selling cars with problems and purposely not telling the buyers about those problems.
There's one more scam that you need to know of and that will be coming soon. Stay tuned.