I've always found vagueness gets the truth better than out and out asking a question. Sometimes more truth than you expect. Because when you're vague, and just imply that you know something, people shit themselves and come clean. Most times. It's not always the best way, but in my experience it works about 90% of the time. Best yet when you know the truth. Out and out asking means someone knows exactly what to lie about. Being vague doesn't. They have to work out what you know and what you don't. They trip themselves up.
The truth will out. I'm normally very good at finding out what I need to know. One way or another.