As several people have pointed out, with just two heads (it's usually two people), the amswer is easy: "If I asked the other head/man what he was guarding, what would he say?" Whatever answer you get, that is what the person/head you are talking to is guarding.
But, as someone has already said, you can't do it with a single question if there the three choices given. It takes three questions. Sadly, I can't remember what they are...
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