We would like to test a lot more but I really don't know how to test some of the critical stuff.

Just as an example, how do you test a parser that processes large amounts of sometimes sloppy semi structured text? Whether a particular defect should be classified as a bug in my parser or as a rare glitch in the source data is undecidable until I know how often the defect occurs.

What I need is a kind of heuristic test framework that makes sure the parser doesn't miss any large chunks that I only find out about weeks later if at all. I cannot supply individual test cases for everything that could possibly be found in the source data.