Right after I listen to their response to "How long have you owned it?", I casually say, "So I guess you're still paying on it?"
They always tell me if they are or aren't still paying on it. If they say no then I know it's free and clear. If they say yes, then I say, "About how much do you owe?" They almost always tell me, about, how much they owe.
I think they usually tell me how much they owe because I've already warmed them up a little bit by asking a couple of questions regarding their situation. I think most of the time the sellers are a bit fearful and nervous about the whole situation so calming their nerves helps. The first question seems pretty harmless, they answer it and nothing happens. So I ask them another question digging a little deeper. They answer that one and nothing happens so they start to loosen up a little bit. By the time I ask the big question they have already calmed down and aren't as tight lipped.
I also think the art is in how you say it.
I usually ask how much they owe as if I'm surprised that they owe anything on it. That brings out something in them to where they feel compelled to tell me what they owe.
It's almost like if you are admiring someone's corvette and you say, "Wow, I bet you didn't even have to pay anything for that did you?" They almost get offended and demonstratively tell you "Heck no, that cost me $40,000 and I'm still paying on it!"