Get an application like DVD Identifier and get the media code from the disc itself.
Once you have the media code, Google it, or go to Videohelp.com and see what other people's experiences are with the same media. It'll give you a general idea on the quality of the media (and you'll be able to find some recommendations on media that people have been having good experiences with too)
Videohelp:
http://www.videohelp.com/dvdmedia (also has a link there for DVD Identifier at Videohelp)