How to distinguish mastery of a complex subject from parsing one formally expressed in a complex way?
In my opinion: the difference between a complex subject and one formally expressed in a complex way is that in the former, the results that you get are really deep (understanding them at the end feels like a spiritual experience).
My experience is that mastery means more like "you have a mental model which gives you 'intuitive' reasons to accurately classify things as true/false and provides some motivation for the reasoning".
An example: you see someone has solved a degree 4 equation by repeatedly applying the quadratic equation, getting 8 solutions. "No way."
Another example: watch a famous baking show and you see somebody put a bunch of different sized pieces of bread in the oven at the same time. Right away: "aren't they going to cook at different rates?" Sure enough, some burned, some raw.