I wouldn't screen for code quality, but for attitude. Every codebase is some level of crap. The older, the larger it is, usually the crappier. This is fixable, however, if they let you. The problem usually arises when the focus isn't on maintainability, the team doesn't want to improve their workflows, or the feature pressure is just too high.

Observe your interviewers: is the interview a questions-answers style process, or are you having a conversation? Do they come over as enthusiastic or rather as close to burnout? Those are tell tale signs of how it will be to work with them.