Ummm, I guess I know what you are talking about but heaps of music store guys where I go are actually nice. There's one place i go to where if you want to try something out, they automatically hook you up to the PRS custom 24 and about foru amps if you want. Well, maybe not that far, but they never care if you pick up the good **** and play it. Also, while they do sit around playing guitar a lot, they never show off, more they seem to want you to join in most of the time.
Hmm, annoying things to test. I reckon that testing tube amps feedback ability is probably going to piss people off. If you walk in, hook up all these effects, then just put the guitar next to the amp and use the tremolo arm a bit while cranking volume might work. Also, random chromatic runs at slow speed with a heavy emphasis on tremolo, vibrato, changing tone nobs, changing picks. Maybe seeing how well you can play their Gibsons behind your head without a strap....