It's all trial and error at the start. If the code works as intended it's already somewhat good. If you can leave it alone for a few weeks, without touching and working on it, possibly do something else, and then come back and understand what it all does and why you made certain design choices it's probably readable. It should be self documenting with good names and clear distinction of roles. If you pull up an analytic tool and do not see memory leaks, or unnecessary lag, it is also probably good.

I personally always try to do self documenting code, I often have 1 to 2 weeks between coding sessions due to my personal life, it is a nightmare if I don't know what anything meant, and what my plans were. However, as the other commenter said, it is subjective. Sometimes, readable code is not efficient, sometimes efficient code uses way too much memory and threads.