Sorry if someone has already said this but you can easily tell if people are traitors by just looking above the chat where the blue "writing in progress" things are, then if they disappear without someone leaving a message they are most of the time a traitor writing in traitor chat.
I have noticed that, but thankfully most people don't seem to go off on that. Either because they didn't realize they could, or just out of good sportsmanship, which you probably can't expect from most people.
However, it could also mean they're talking in Detective chat, which is like- the opposite of a traitor.