California didn't originally detect this. According to the letter from the EPA [1], it was a team at West Virginia University doing a study that discovered that TDI's in normal usage were found to have significantly higher emissions. California's CARB then dug into it to confirm their results and work with the EPA to figure out what was happening.

[1] http://www3.epa.gov/otaq/cert/documents/vw-nov-caa-09-18-15....