Back when I worked at Boeing, I was once compared poorly against another engineer who was always down on the factory floor when called, any time, whenever there was a problem with his stuff. The thing was, I was not called down to the factory floor because my stuff worked.

I'd say a developer is doing a good job when you, as a manager, are not having him come in at all hours to fix problems with his stuff.

Other signs of a good developer are you don't have to micromanage him, and he's more of a partner helping you get things done rather than a subordinate you've got to spell things out for and check up on.

I.e. when he's on the job and you can relax and concentrate on other things, you've got a good 'un.