The one exclusive sign of thorough knowledge is the power of teaching.

You can hold something well enough to use it and not well enough to hand it over. Teaching exposes the gaps immediately, because explaining requires you to know why each step follows and to find the place where somebody else’s confusion actually sits. Until you have done that, the feeling of understanding is untested.

Use it as a check on anything you claim to know. Explain it to somebody with no background, out loud, without notes, and pay attention to where you get vague. The vague part is the part you do not have yet.