Learning Adventure #2:Toggling Thieves

You've arrived at the port city of Arsesia. From there you head towards the court of the city, where your next task awaits you. Once you arrive, a man asks if you are the student the court had called. You reply yes. 

He tells you that he is an assistant of the ruler of the city. You ask him what's the issue? He explains that the city museum's diamond was stolen by a group of thieves. The Oracle of the city told that there were 4 people in the group of thieves, and narrowed down the suspects to 5 people. The magician of the city cast a spell on the suspects that caused the thieves to "toggle" and the person who wasn't a thief to always tell the truth. When people "toggle" the first time they're asked a question they'll either tell the truth or lie. The second time they're asked a question they'll do the opposite. For example, if the thieve tells the truth the first time, he'll lie the next time.

Then the assistant asks you, what 2 questions can be asked from one random person from the group of suspects to find who's the thieves?