So you're on a team using all of the agile techniques (please don't say methodology). You're pairing, using TDD, daily standups, etc. You have the maturity to understand (or you've been on the other end too many times) that micromanaging doesn't work, but you want to keep a close eye on one of your team members; the code she writes often ends up being less than healthy. In fact, it sometimes is beyond refactoring and needs to be rewritten. You'd rather she not spend a great deal of time going down ratholes. What do you do?
Rotate pairs often. This is a stealthy/healthy way to keep fresh eyes on the problem. It's the classic agile way of spreading the perspective around.
Dope a rope. This is a technique I use sometimes to be welcomed into another developer's sandbox. "Dope"... means I express ignorance (maybe too strong a word) about what he's doing and ask 3rd grader type questions. (Easy for me now - as I have no developer credentials on my current project and am acting as a project manager). I ask for explanations that lead the developer to "throw me a rope" and pull me into the code to show me.
What is the result? The developer sees that what he's doing matters (because I'm paying attention) and feels confidence (after all - he knows more than me). I get an opening to subtley convey suggestions for improvement. And I usually learn something.