There is research and due diligence, and then there is seeing things firsthand.
Dubas absolutely did his research. He probably looked at past scouting reports of what the Pens had in their organization and at the NHL level, used contacts to learn what others thought of the coaching staff, etc.
We do the same when we're looking at jobs, right? Ask around, look for red flags, see who you know who might know something more than the HR folks are telling you.
But there is nothing like getting in and seeing things yourself. And I can count numerous times in my own life when I've joined an organization or taken a new job and had been told by outsiders, "Yeah, it's a good place, but Person A has their fingerprints all over it and screws it up," only to find out Person B is the one making the decisions and Person A is the scapegoat. And, of course, sometimes Person A does just suck.
And that's why you do your own evaluations, especially as a manager. Do your research, be prepared, and then let people show their true selves after you're in position to make decisions.