That's a good question. The early warning sign is the safety system.
The new attempt by the CFIA to get early warning of impending problems--that is, buildup of these organisms on the equipment and growing in the crud that occurs in the bearing traces and in the joints inside the machine--by swabbing the food contact surfaces on a regular basis will provide a history. Over a course of months--and correct me if I'm wrong, Ron--you'll see in a food plant....
You have maybe ten machines working in a row. If you're monitoring all ten of those machines that make hot dogs, and you see a problem developing in one of them, which shows listeria, then you know that it's time to take that machine out of the line. At the same time you identify a problem you start testing the end products. Don't misunderstand me. I'm not saying don't test the end products, but don't test the end products in a random fashion.
It's the food-contact-surface swabbing program that is the early warning.