The Truthsayer can answer any question. State your question, press the button, see the answer.
Here is the trick that makes the truthsayer always say "the truth": Press at the right of the button and get a positive answer, press to the left and get a negative answer, or press at the top to get a neutral answer. The rest of the button will give a random answer. It is a good idea to rehearse a bit before you show it to your friends.