You measure quality in code the same way you measure quality in science, through peer review.

In .Net development (and I'm sure in other languages/platforms) we have static code analyzers that can help by pointing you towards potential problems. But the best way to ensure good quality code is through code reviews. These can be quick and informal like "Can you take a look at this before I check it in?", or they can be more formal where and reviews a piece of code that one person wrote.

The group reviews can turn ugly, and that needs to be kept under control, but they can also be educational as everyone gets to see what other people are doing that's right and wrong, and hopefully clean up in their own code as a result.