I will too. I'm in favour of using our senses and our brains. Not just looking at the expiry date. If the food is spoilt, throw away. Nevermind if it is before the expiry date. If the food is not spoilt, don't throw away. Even if it's past the expiry date.

The expiry date is for you to use in the store. Before you buy it. It's harder to tell if the food is spoilt when it's still in the packaging.

How they determine. By testing. Manufacture a small test batch the same way, store in the lab, and open some up after 3, 5, 7, 10, etc days. See if it is spoilt. For very long expiry dates (eg years), you don't have to wait that long. Can measure the level of "spoilage" and extrapolate using biological math models.

BTW: I'm just guessing. I do not work in the food industry.