No matter what level of skill you're at, you can start looking at the software you use day to day and think "Could I build that? What parts would be easy? What might be hard? What might be tedious?"
Try that with a few different project ideas. There will probably be some tools/techniques/skills in common between them, some of which might be ones you have some skill with but don't feel fully confident.
From there, you have a topic of study, and can go dig deep into books, exercises, practice projects, etc. (Or find someone to mentor you and give feedback)
You'll know you're doing it really well when either (a) you're fairly certain that no one has a much better way of doing it than what you've already learned... and you've searched far and wide and come up empty or (b) the area of study becomes easy enough and automatic enough that you don't need to give it much thought... it just becomes automatic.
You'll know when you have something that can be improved when it distracts you away from the problem you're solving... repeatedly and not in an "essential struggle" sort of way, but a "I really wish I was better" at this kind of feeling.
Some level of proficiency is necessary to recognize these pain points and fix them. And the solution is usually to develop more competence, so long as it's in an area that really does matter to your work at hand. These two things work best as a spiral, supporting one another.