Unfortunately, I think the best barometer to measure your growth as a developer is looking at your old code and cringing. This morning I needed a little enhancement to a web page, and rather than writing something new or looking for something to download, I went and pulled out an old script I'd written 4+ years ago. It's perfectly functional and worked on the first try, but I made the mistake of looking closely at the code and almost wanted to puke.
I've almost always gone through a wave of "man, I wish I'd done that differently" headshaking after significant projects. I've never been able to look at a completed system six months later and not see ways that it could have been built better or simpler. If your code from a year ago looks good to you, you might just have peaked as a developer.