There are basic tests. Turn one of the output flange, and the other should follow. Hold the input flange and one output flange and it should be very hard to spin the remaining output flange.

It can fail or wear out from abuse or misuse. That's why some people like the idea of getting the same vlsd out of J30's. You hardly ever see J30's getting abused. Like Funk said, having different sized rear tires will destroy it as well.