This might not be true given just a couple of hours staying in the office and talking to people. Management issues / technical debt are not visible until you work for the company.
The best I've come up with is a few proxies, like their deployment process. Someone doing continuous or frequent deployments probably have pretty decent code quality. A company that takes several weeks, several rounds of QA and a dozen release documents that need to be signed in triplicate, they have that process because they've been burned and need scapegoats when the inevitable happens.
The best part is that most people at companies like this aren't aware that it's not normal, so they'll be open about it.
Yeah, that's basically all it takes. Talk to engineers about their everyday processes. (If the company doesn't let you talk to their engineers during the interview process, or if they're unwilling to discuss, consider that the huge red flag).