Hurm that's prety neat. Down here we just have bottles or cartons never seen the bag before. As for useby dates, I assume it'd be stamped on the bag, or whatever the bag comes in?
But assuming the milk bag is put back in the fridge after each use, it would have the same shelf life as normal milk. And I'd say it'd be prety easy ASM to tell if it was off, by the rancid ass smell and chunks floating in the bag.
-N