There needs to be some type of measurement, both objective and subjective.
Part of it is that there are different directions. I'm thinking about progress, but also about ease. I'd expect the same level of ability to become easier if I'm making progress.
Maybe the simplest....
Pick some aspect.
Pick a goal/direction.
Focus on something that should improve it. You should see some progress.
For learning (and probably anything) sleep helps. Things need to be consolidated.
Timewise, you might see results immediately, the next day, two weeks, or even six months. If after six months, there's nothing, then it's pretty certain whatever you're doing isn't working.
The method for improvement is the part where a teacher is helpful. Except a teacher isn't always around or the area might not be known.