“As a group, please tell me three facts about <the topic.>. Raise your hand if you have a fact to tell me”
If someone says something that you suspect isn’t true, still write it up and don’t immediately tell them that they are wrong. Ask whether anyone else in the group is of a different opinion. If no-one else says anything, don’t worry about it. Hopefully they will work it out by the end of the session. Just move on.
People like to show that they already know something about a topic. It helps you get an idea of their prior knowledge.