Wah... u guys are so precised..
in my personal humble opinion, there's no fix stuffs that you'd play to really 'test' that guitar before buying
let's face the reality. half of the time is about showing off
no? let's be honest with ourselves haha...
anyways, i love to see and watch people trying out guitars at shops. in my 19 years of playing the guitar, I don't really have anything specific to test the guitar. On technical part, I'm no guitar technician, but of course I'd check for neck alignments, and cracks/knocks/chipoffs/etc, some minor test on every pickup to make sure the swtich is working also for the knobs, the machine heads, well jus a bit here and there. And in my experience in buying my guitars in the past, I've not had any problem after buying, except for a cracked body on my Jem which was the biggest blow i had.
depending on what guitars I try, 80% of the time, I know what I'm going to buy the moment i step out of the house. Any seasoned guitarist should know that too. If you are buying a strat, do expect a strat sound, the only difference would be the country of manufacture, and also the kind of amp used for the 'test'. If i'm out there to get a Ibanez (modern guitar), then, I should have a basic idea of the sound i'd be expecting. right?
for neck test, i'd do some modes, from the first to the last fret, some and chromatic scales (my best choice to test fret). i dont have to do the scales fast if my sole intention is to really test the guitar. If someone purposely stand in front of me to test me, then i'll shred. Else, i be my natural self, the humble guitarman.. anyways, i have nothing to show off, no skills no need to show off lor.. haha!
oh.. songs? no specific songs. i dun need it, just do all the modes, thats all. for strats, i love the twangs, so i'll do some slow blues pentatonics. this way, i get to hear the twanging sound that i love, plus get to feel the guitar. 95% of the time I get the assistant to test. And that was how I found my personal guitar teacher (he was a lead guitar champion in one of the lead guitar competition.. good rite?
Who knows one day you found your personal guru that way too.