I just go to wherever the FM units are distributed and ask them for one. If the person working there doesn't know me, they may ask why, in which case I just pop out my hearing aids (they're nice and small, fit way in the canal so you can't tell I'm wearing them unless you look closely). I'm fairly young (graduate student), so I'm sure I get asked for proof more often than someone older. My brother is an audiologist and has his own FM system, so he just lets them know he'd like to use his FM unit and asks them what frequency they're using. If they don't have an ALD set-up, he brings his whole system and sets it up with the sound guy.
As far as sound quality goes, it depends on a million factors. You have reception quality to think about. I believe the source is most often from the SBD and sometimes from an overhanging mic. Some venues mix them as the show "should" sound, while others pump the treble way up since most losses are high frequency. With hearing aids in and using an ALD, the sound should be well balanced, as a good recording would be, but some people still insist on pumping the treble up. I guess that works for hard of hearing people who don't have hearing aids. But, like I said before, I usually just use them for between song banter unless it's a huge stadium venue (in which case I ask myself, "Why I am seeing a concert here?").