Unfortunately, it's really really difficult to test for randomness. We can test for particular patterns, but nowhere near what a determined human with access to the source code can find.

At the moment, the only "correct" test I know of is hilariously impractical: enumerate all possible programs up to a reasonable length, run those programs against output from the random number generator for a few minutes each, and see if they're better than chance at recovering the seed you used.