The key is to test the cap seal. For awhile there you could just look and see that the seal had been broken, and then I tested the cap on one jug that looked okay, and it wasn't. Turns out the thieves had screwed the cap back on tight, and it just LOOKED like the seal was unbroken.
So the technique is...check the bottling date...now that the promotion is over, you can't use the receipt, then you begin to unscrew the cap. It should free up for just a nanosecond, and then you begin to feel resistance...that is a sealed cap. The ones that have been screwed back on, just as it frees up it looses all resistance and begins to turn. That's an opened cap that has been screwed back on tightly.
This one almost caught me the other day, thought I had found another stash, one was missing its cap entirely, others had the snap ring broken, but in the back were what looked like unopened jugs. So I tested one, just put a little pressure on the cap and it just spun right off. Having removed the caps from the 20+ jugs in my garage, I know exactly what the resistance from an intact snap ring now feels like. S, I find that, and I know I've got a good cap and jug.
Have been focusing on PP lately, 10w-30. I have a shear problem, and the 10w-30 is holding up better than the QSUD 5w-30 I started with.