Cool setup! How do you check false positives? You take a look at the spam folder for legit emails?
Checking for false positives is a manual process. I've never had legitimate mail marked as spam though, only the other way round.
My experience with Gmail's filter is not as nice btw. At work, I had a couple of situations where important mails from a customer were flagged as Spam, which went unnoticed for two weeks.