In a code review, what do you look for to assess a developer's expertise? Given an opportunity to look at a developer's work on a real-world project, what tell-tale signs are a tip-off to carelessness or lack of experience? Conversely, where do you look in the code to find evidence of a developer's skill or knowledge of best practices?

For example, if I'm looking at a typical Rails app, I would be happy to see the developer is using RSpec (showing a commitment to using test-driven development and knowledge that RSpec is currently more popular than the default TestUnit). But in examining the specs for a Rails model, I see that the developer is testing associations, which might indicate a lack of real understanding of Rails testing requirements (since such tests are redundant given that they only test what's already implemented and tested in ActiveRecord).

More generally, I might look to see if developers are writing their own implementations versus using widely available gems or if they are cleaning up code versus leaving lots of commented-out "leftovers."

What helps you determine the skill of a Rails developer? What's your code quality checklist?