Unless someone is listed as a reference, you're not allowed to ask about job performance, reasons for leaving, etc. You can only contact to verify that the person did actually work there and when.
Even with a reference, employers will often be reluctant to dish actual dirt, for a variety of reasons, from pity to legal liability. You have to read between the lines and be up on your euphemisms.
You can ask, they may not answer. One good question that can often ferret out issues is "do you have any advice on how to manage him to bring out his best performance?"
I think your suggestion is good. But knowing people they're going to be pretty diplomatic about their answer as well. Spitting out generic schlips that you already know. I guess the best advice is Iron_ball's. Just try to read between the lines, but thinking about it, even that method is a stretch :-(