I was considering it as an open question not a yes/no. If they say yes you ask them how. That way they are demonstrating their knowledge rather than claiming it.

Like if you wanted to find out if people know that bacon flavour crisps are vegetarian you'd ask something like "do any crisps contain meat or animal products?" and be prepared with follow on questions. So if the response is "no" you go back with "not even BBQ beef or bacon flavour?". If they answer "yes" you ask "out of these, which are suitable for vegetarians: ..." or somesuch.