The litmus test for me is if I could explain, in words and with little mathspeak, what the question is trying to ask and the strategy you've adopted to solve the problem. Which means I often speak out loud to thin air - perhaps a trait of a weirdo, but necessary.
To put it anecdotally, I like to approach understanding maths (or anything, really) as if I'm a storyteller, except the story here is perhaps a bit technical, and your audience is perhaps a smart 5 year old who's quick to understand things but lacking the base knowledge.
Though I should stress that there are always methods that are so mechanical that the ability to execute trumps understanding. For instance, knowing how to apply cosine rule is much more important than understanding why cosine rule works. This can only come from repeatedly doing questions over and over to build up muscle memory. In fact, that's my philosophy when it comes to learning stuff - know how to do the thing first, then comes understanding, then comes storytelling. It's perfectly fine to do stuff that you aren't entirely sure what's happening - give it time.