Drywall Guest Room
Another common sign is your home may contain defective drywall is the tarnishing or corroding metals and metal wiring in your home. Copper wiring that would normally be shinny becomes corroded by gasses emitted from the drywall causing the wiring to become a dull black.
You can look for evidence of corrosion by carefully removing a faceplate from a wall outlet and looking to see if the ground wire, usually an exposed copper wire, has been turned black.
Additional clues can be seen by looking at the air conditioning coils of your air conditioner. In many homes affected by defective drywall the air conditioning coils have lost their copper shine and been turned black due to the corrosion caused by the drywall. Many homeowners have replaced air conditioning units several times just to have the new unit fail again in only a few short months due to corrosion of the coils.
Chinese made drywall also has a slight variation in color from the more commonly used American made drywall. Chinese drywall has slight gray tint to it while American drywall is whiter in color.
Many homeowners have also noticed tarnishing and pitting of metal fixtures throughout homes that are believed to contain Chinese drywall.
More information at wikipedia
