Skeptical.

That doesn't mean hostile, though. I mean skepticism in the sense that nothing is taken for granted and that all is fit to be questioned. Only tangible evidence in documents, specifications, code, and test results matter. While they may patiently listen to the reassuring, comfortable words from the programmers ("Trust me. I know where the bugs are.")-and do it with a smile-they ignore all such in-substantive assurances.