The best thing to do is to ask the seller if they have a picture of the back of their item - especially if it is an expensive piece because they only increase in value according to the quality of the stitching.  The back should look basically like the front!  There should be no knots,  not across the piece straight strings.

That is the first sign of quality. Then make sure that they have prewashed the thread so that the colors do not fade into the material over time.

If the item is framed is a non-acid product behind the stitching.  A non-acid product keeps oils from seeping onto the cloth causing stains.

http://stores.ebay.com/LVG's-Stuff-and-More-Stuff