The ability to explain something simply usually follows understanding it deeply.

Simplicity in explanation comes from understanding, not from talent for explaining. When you genuinely grasp something, you know which parts are load-bearing and can leave out the rest. Complicated explanations often signal that the speaker is reproducing rather than understanding.

Use this as a check on your own knowledge. Try explaining something you know to someone with no background, without jargon, in a couple of minutes. Where you cannot, you have located a gap. That is more useful than any test, and it points precisely at what to go back and learn.