There's a great searching tool available that lets you track individual artists, labels and albums.http://riaaradar.com/search.asp
A few results of labels to avoid when buying new CDs:
Roadrunner (tons of death metal, basically anything recorded in Morrisound and produced by Scott "Studio Vampire" Burns; no wonder they whore out their back catalog so feverishly these days)
Def American (Slayer)
Sanctuary ('Sabbath, Motorhead, Megadeth)
Megaforce (Overkill, Testament)
Noise (lots of great speed metal like Coroner, Kreator, Helloween, Rage along with modern crap like Dragonforce and Kamelot)
Caroline (Misfits, Suicidal Tendencies)
Noise being RIAA is a surprise and major bummer; they have tons of awesome and foundational metal albums in their catalog.
It's funny how the American speed metal and death metal movements were funded almost entirely by the RIAA.