Hand the system to someone who has never heard the reasoning behind it. A new colleague, an agency on their first brief, a developer building a screen nobody discussed.
How do you know a brand system will hold?
Let them make something real, then look at what they produced. Not at whether they liked the guidelines. If the result belongs in the system without a round of corrections, it holds. Every correction you have to make is a rule that exists in your head and nowhere else.
Do this before the system is finished, not after. A rule that survives one stranger will survive the next forty. A rule that needs explaining will need explaining every time, and eventually nobody will ask.