The problem with great managers is that you don't notice them. Things just work, no drama, no fires to extinguish, ... . It's also not clear why things are going that smooth, it just seems to work that way (clue: it's actually the manager that creates this environment).

I've worked in a few environments where managers were replaced. Only then can you really make the distinction between great managers and bad ones. With one you think "wow this is such a tough business, the manager is doing crazy effort to keep things going", and then another manager jumps in and "wow, this is an easy project. Manager is taking a walk through the park. All seems like a piece of cake."