How best to statistically verify random numbers?
You can't.  Randomness is a property of the source, not a property of the values you get from that source.  In other words, randomness is a statement about the probability distribution, not about some specific values sampled from that distribution; from a finite sample, you can't give a definite answer to your question.

Or, to quote Dilbert:



What you can do is perform some kind of statistical test, with the properties that (a) if it is truly random, it'll probably be accepted, (b) if it is not truly random, it might be accepted or might be rejected (no guarantees).  In other words, if the statistical test rejects it, then odds are it wasn't random.  However, if the statistical test doesn't reject it, you can't conclude anything -- it might be random, or it just be a good enough mimic to fool that one statistical test.  Such statistical tests are often sufficient that they can reject many obviously-bad generators.

If that sounds like the kind of thing you're looking for, look up the DIEHARD test suite and related tools.

See also How can it be detected that a number generator is not really random?.