Test-driven
How much test code a developer writes alongside the code that runs the product.
What it tells you about a developer
A developer who tests their own side projects, where nobody is making them, has made quality a habit. Expect fewer late surprises and an easier review culture around them.
Tests are code that checks other code, and writing them is a professional discipline that pays off in fewer production failures. A developer's ratio of test code to product code across their repositories is a rough, checkable read on that discipline. Personal projects run lower than employer work here, so the pattern needs to show up across several repositories before it means much.