Detecting managerial quality up-front is a rather hard problem. I believe it's mostly rooted in the fact that anything meaningful you do as a manager plays on a long timescale (months & years), so you can't directly verify. And it's sufficiently fuzzy that in a short conversation, managers can lay down enough smoke screen to make it hard to see flaws.
So, what you need is getting some kind of long-term view on that manager. There are a couple of ways:
* If you can get it, data. Retention/Turnover, how well people grow, internal surveys. That's almost exclusively available if you transfer within a company, so limited usefulness.
* References. Find people who worked for that manager and ask them. Even better if it's ex-reports, since they tend to be more frank.
* Are they listening? When you have a conversation with them, are they paying close attention to what you have to say, or are they busy delivering a message? You want listening :)
* Can they communicate clearly? Are they able to express what their team does, and what their main challenges are, in a coherent way? In less than a minute? Communication is a managers main job. If they fail at that, you'll have a horrible time.
* And finally, gut check. How do you feel around that manager? If you're in any way uncomfortable, see it as a warning sign. That doesn't even necessarily reflect on their skill as manager - there are just some people we're feeling good around, and some who make us miserable. Skip the miserable part. (If you have a skilled manager, they can minimize the misery somewhat, but you'll still both be miserable.)