How to distinguish quality used furniture? (ZT)

Try moving it around. Do the chairs wobble? Good quality chairs don't. Table legs can usually be tightened. Beds are notorious for being loose, but if they use old fashioned bed bolts, they can be tightened up. I usually equate good quality furniture with being sturdy enough for its intended function (and the very best uses construction techniques and fits that maintain their integrity for generations).
Dovetails are a good construction technique. Raised panel door construction is another. Mortise and tenon joints are strong and durable (and sometimes may be given away by wooden pegs driven through the tenons). If you look at tables with leaves, look at how the slides are constructed - the heavier and beefier the better.
Look at the secondary woods, and how it is fastened and finished. Since this doesn't show, this is an area where a lot of lower quality goods may cut corners. Any secondary wood that isn't "real wood" would make me question the quality (though I have seen some very nice Baltic birch drawer boxes).
Look at how the pieces are fitted together. On good quality pieces, there will not be any gaps or fillers. You also won't be able to easily tell where they have filled any nail holes.
Finally, look at the visible wood. I like solid wood, because it is easier to repair and finish (look for continuation of end grain around the edge of tops to see if it is solid). But don't discount veneer - some of the best furniture uses veneer (just make sure it is firmly attached, smooth, and flat). Good quality furniture doesn't use finger-jointed lumber, nor will you be able to detect where surface defects have been repaired. Flat surfaces will be flat, not rippled like someone is trying to mimic hand-planning by doing a poor job at it. The finish should be smooth, without dust, runs, orange peel, or splotchy areas.
Note that it is a lot easier to hide poor workmanship or wood under a dark stain. On the other hand, if you are into refininishing, sometimes you find diamonds under layers of old paint or varnish.