There isn't a formal mechanism you can use to determine if folks have a SWZ, generally you just request the file and the Player will load it from the cache or go ahead and load it from the location specified. So you could write a test that simulated that, you'd basically provide an invalid location for the file and if it the load was successful you know the user had it cached and if not then you know they didn't have it. We don't publish where the SWZ is on their computer. And there is a set of SWZs for each version of the framework, so age doesn't really mean anything, it's all about what version you're targeting.
We don't have published stats on SWZ usage, but they're getting real use despite not being enabled by default. At a level where we think folks should feel OK about having it turned on.