Anyone who wants can participate in a single or double blind listening test. There are no requirements that I know of.
Of course a philosopher could argue that anyone who actually does want to participate in such a test probably has preconceived notions that might or might not introduce a testing bias. By this thinking, the best participants would have to be unaware of the true nature of the question involved.
It's my impression, based on reading Toole's book a few years ago, that blind listening tests of loudspeakers using experienced or inexperienced listeners produced little difference. If anything, experienced listeners performed slightly less reliably than inexperienced listeners.
Sorry for the late answer.
Swerd
I am involved in reading Toole's 3rd edition book right now and thankfully I have covered the testing scenarios section. What he said about experienced vs inexperienced users is that inexperienced users can be trained to listen critically in very short order. In other words, listening aint rocket science and with a few helpful hints and tips a relative newb can perform as consistently as someone who is an experienced listener in relatively few sessions.
Can you "want" to participate in a DBT without fouling the test. Of course you can. It happens all the time. Without people who want to listen critically to audio the Harmon testing center would be screwed. The genius of the DBT is neither the testor (the guy or gal running the test sequences) nor the listeners know what is being measured and tested. You can be gung ho as hell about being part of the test, but, you will not know what you are testing or listening for or what variables are being changed behind the blind curtain.
In a true DBT, you don't know if it is source music that's being tested, speakers, amplifiers, cables, connectors, or any of 100 or so things we can do audible tests for. You can't see what's being tested and you don't know the order or even if things are being changed behind the curtain.
If more people who post on this forum about their golden ear ability to discern between A-B testing understood how flawed a method that is, they wouldn't be crowing so loud about how they can hear the difference between a 16bit DAC and a 24 bit DAC when they can see both and know which one is playing and that's what is being tested.
anyway, that's my two cents. I am loving Floyd Toole's 3rd edition book. It should be required reading material. For whom, I'm not sure. But it should be required reading.