How do you know that a quantum source is random?

Building a true random number generator is (theoretically) easy with quantum mechanics. But if your arch-nemesis gave you a device which he claims produces random numbers, how could you verify that he speaks the truth?

We present a randomness verification method based on quantum xor games, initially developed by Roger Colbeck and extended over the past few years. We then show how a small amount of initial randomness can be used with untrusted quantum devices in order to produce high min-entropy strings of unbounded length.