1. if by some miracle you are facing a wall of guqin go thru them one by one and put one hand clamped on the forehead and then pluck strings 1 and string 7 - tend to pick the ones that have the most vibration.
2. if you know the notes play them all (take your time) on all the strings from hui wai to hui 3 or 4 and listen to the sound and listen for buzzes. If you don't know a note from a hole in the wall - play the hui positions.
3. hold down string 1 at both hui wai and hui 5 and look for *light* underneath it at about hui 7.5. If there is no light - don't buy it unless the sound is fabulous even then I myself wouldn't buy it. Fretless fingerboards need a certain amount of concave-ness under the strings. You should see some light. No light -> more buzzes. Some guqin makers unfortunately don't know about the camber thing but most do.
4. if you are a true nerd you would show up with a ruler that can measure mm - so make sure that at hui 7 the strings are not too high - you should expect that the distance between the top of the qin and the bottom of the strings (doesn't matter what kind of string) is <= 7 mm maybe a little higher on string 1-2 but not much. The Wang Peng guqin I have measured are around 5-6 mm. 5 mm is possible (actually at hui 7 you are measuring a bit of the camber too but oh well - that's fine).
5. last my father-in-law has a rule for buying stuff in general which is called the "cheap charlie" rule (by him) - don't buy the cheapest one.