[phelmig]

How many problems do you miss until you start debugging? Getting complex models and systems right in your head is a matter of training.

Also: How good is your documentation? The more experience you gain the more you will be able to plan ahead and build the a scaffold first before you start fiddling with details. This often leads to well documented code where the documentation always matches the code. A lot of tinkering often means that the documentation and your code won't match.

How well do you know your tools/frameworks/language? Can you start coding or do you find yourself trying out something in a REPL or a small test project before implementing it? (Note: This may vary since even quite senior developers have to learn new tools)

How well can you explain your code / your program / your project? Once again getting a good grasp at complexity is a matter of training.

Compare your code to open source projects and other code you can find. You'll see that there are always some good practices one still can learn. On the other hand becoming more experienced will make you see code and decide that there is room for improvement or you would have structured it better.