For more expensive guitars, you can really bring out the white glove and get fussy. Prospective buyers even bring in a dentist's mirror to inspect the interior of an acoustic guitar.
For acoustic guitars in the mid-priced to expensive range, you should expect to find gapless joints — solid wood-to-wood connections between components, especially where the neck meets the body. You should also expect clean and glob-free gluing (in the top and back bracing), a smooth and even finish application, and a good setup: the strings at the right height with no buzzing, the neck warp- and twist-free, and the intonation true.
Look at the places on a guitar where different surfaces meet — particularly where the neck joins the body and the edge of the fingerboard where the metal frets embed into the fret slots. You should see no trace of excess glue, and the surfaces should be uniformly mated to each other.
You can glean all this information by simply playing the guitar and noting your impressions. Like traveling in a Rolls-Royce or Bentley, playing a quality guitar should be one smooth ride.
