What criteria do you use to test the quality of a bag?
I check the stitching (make sure it's even, solid and not fraying), the quality of the leather (evenness of texture, thickness, grain, smell), the security of the handle/strap attachments (does it look like they might begin to pull away from the bag over time, or is the weight distributed evenly such that they remain flush with the bag), security of the hardware attachment (is it loose at all, will it fall off or break with use), and the overall balance and feel. A lot of this stuff depends so heavily on the style of bag that it's hard to make catch-all pronouncements. For example, on some bags thick, squishy leather with an uneven texture is what you want, on others you want a smooth polished appearance with a very fine grain. I think stitching is my #1 criterion for quality, as it's what holds the bag together!
Ah...so many things. The main things I look for are:
1) Quality of the main materials - is the leather soft, is it strong or fragile? Is the leather dyed or just painted? Is the metal hardware finished evenly, without chipping?
2) Quality of the construction - No glue on the seams, even stitching, hardware attached securely, zipper works smoothly, etc.
3) Style of the bag - will it go out of style in 20 minutes, or is it a classic? Will the color go with everything, or is it funky enough to have very limited use? Any of these are OK, but they are considerations, especially if it's an expensive bag.
4) Design of the bag - is it balanced, or does it sag in the middle? Can it be opened comfortably or do you have to squeeze your hand in to get something out?
The primary issue is how the leather/other material is cut, stitched and finished. Quality leather isn't as expensive as people think, but it does cost to cut the material well and then stitch it up properly.
The good thing is that quality isn't tied to price, and if you can stay away from the seriously trendy bags, or the price gougers, then you can get a great bag for a much better price.