First and foremost the instrument should have a very pleasing tone so I would try as many as possible until I found a tone I liked.I would check that it has a solid spruce top on it and not a laminated top,I would check that the frets are perfectly seated in the fingerboard and that the fret ends were smooth with no ragged edges.I would check to see what the fingerboard material was made of (ebony preferred,rosewood is usual).

I would check that the instrument plays in tune all over the neck,if I couldn't play it I would bring someone who could and ask them to play it for me.I would check that the machine heads for tuning were of good quality and worked smoothly without slipping.I would check that the finish was not flawed, that the nut was correctly slotted and that the neck was not warped.

I would enquire about the wood types the instruments back and sides were made of(solid or laminated wood,maple or mahogany etc)If inlayed I would check that the inlay was perfectly smooth with no protruding edges and no glue visible.I would check the inside of the instrument to make sure it had a tidy construction e.g. no glue drips or splintered linings etc.