I'm probably the worst guitar shop player ever
Whenever I pick up a guitar in a shop, people must think I'm the rawest of beginners. But that's ok - I'm not there to impress anyone, I'm there to thoroughly test out the instrument I'm considering buying.
For me such a test will always be fairly systematic, and not necessarily pretty sounding. You need to test/inspect as many aspects of the instrument as possible, so you don't get any nasty surprises when/if you take it home. I would probably do something like this:
1. Strum some simple chords, for example an open A. Just a single strum, then let it ring. Really, really listen
to how that chord rings out. Does that sound speak to you? I do this acoustically even with electrics, because for me personally the basic acoustic tone is so very important. You're looking at an acoustic, so of course this is also important to you
Just be aware that string brand can make a world of difference on an acoustic guitar...
2. Tuning. Before even starting to decide if I like a guitar or not I'll be checking out how it holds tuning. This is the number one deal breaker for me, if a guitar has tuning problems. Doesn't matter how it otherwise sounds..
Tuning problems can be many, and some are very very important to check especially on cheap guitars, because they are caused by some of the ways manufacturers cut costs - hoping that beginners won't notice, because the cheap components often doesn't look
different from the more expensive ones.
a) Machine heads - do they hold? Tune up the guitar, do some wild bends (if your fingers are not strong enough, simply pull up the string rather forcefully). Check if the strings go flat. If they do it can be because it's new strings. But if you tune up, pull, tune up, pull, tune up, pull... and they are still flat. Then perhaps the Machine heads will cause you problems.
b) The nut - now this is a quite tricky one, that many beginners will have a hard time spotting. Is the nut cut correctly? The usual (and common) problem can be this: tune up the open strings so they are perfectly in tune. Now play an open E chord. Check each of the fretted notes against the tuner (B,E,G#) - are they
in tune? They should right? But they aren't always, if the nut is cut incorrectly. If they are not, try tuning up the string so the open E plays correctly. Now fret an open D and check those fretted notes (A,D,F#) - are they now in tune? You'll find that with a poorly cut nut it might not be possibly to reach a tuning where both of those open chords play perfectly in tune! All open chords should of course ring out perfectly in tune
c) Does the strings stick in the nut slots (if they do, this can at least be fixed somewhat easily)
d) Check intonation (can also be adjusted, so not a deal breaker if it's a bit off...)
3) Check the fretboard. Check if there are any fretted notes that causes a buzz. If there are, again it might be possible to fix with a setup. But could also be an indication of poor/uneven fretwork overall.
4) Sharp frets at edges of fretboard? When you glide your hands all the way up and down, feeling both the top and bottom side of the neck - do you feel the frets sticking out?
5) Sharp frets on top of the fretboard? Very hard to test in a shop, but if the fretwork is too cheap the frets might not be rounded enough and have sharp/rough spots. This can cause strings to break if you bend near a sharp fret. Very annoying problem...
.... and of course - just play the guitar for a while and see if you enjoy playing it