How can I verify that a PDF file is "good"?

In similar situations in the past I have successfully used the PDF Toolkit (a/k/a pdftk) to repair bad PDFs with a command like this: pdftk broken.pdf output fixed.pdf.

OK, what I ended up doing was using itextsharp to loop through all of the stream objects and check their length. The error condition I had was that the length would be zero. This test seems quite reliable. It may not work for everyone, but it worked in this particular situation.