I think, a good indicator is to look at old code. Use code which is about a year old and compare it to your current architecture. Try to see what you have changed and why you think your current code is better. You will immediately see the things you figured out over the past year! The code you write is always a reflection of your current level, so there is no better scale than the code itself.
When you get good enough to take a look at code you wrote a year ago and don't think "Ah, what did I do there, that's so unmaintainable, I will have to rewrite everything to get the changes done", then it's a good indicator that you grew a lot. Well, even I think that I might write better code today than back then, but imho, it was pretty ok back then and I feel confident to make a change and release the old code as-is without worrying too much about other people (other professional devs and veteran FOSS devs) seeing it.