Other than looking at the back of the drywall for a label (in the attic) or looking at terminal screws for black discorloration at outlets and switches, is there anyway to be certain about the possibility of the presence of the bad drywall beside destructive testing?

[Bruce A. King]

no, not really, and taking physical samples for testing would technically require a sample from every sheet in the house since more than one shipment could have been delivered to the site during construction.

The A/C coils are another good place to check for discoloration.

[Nick Gromicko]

http://www.nachi.org/chinese-drywall.htm

[Blaine Wiley]

In addition to what is listed on the INACHI drywall page, the evaporator coil of the air handler should be checked. I found Chinese drywall in a home last week only because I saw the blackened copper fittings on the evaporator coil. Yes, opening the air handler is beyond the scope of a normal visual inspection, but if we're inspecting for indications of the presence of Chinese drywall, it is a must. It turned out that only a few sheets of bad sheetrock had been used in the home, contained at a small area of the kitchen and adjacent foyer and laundry room.

In addition to destructive testing, sheetrock can be checked with an XRF gun (X-ray Fluorescence), which is the same instrument used to properly check for lead. This would be the preferred method, although the guns are very, very expensive and few people have made the considerable investment to purchase one and become properly trained.