The seals on jugs were big wood plugs, just like now (or rather, just as recently), but it’s possible that the contents can be figured out. Of course, they probably made more than one trip, and had different contents at different times, and some of those contents may have left more of a trace.
some of them might have been buried just right and the wood petrified with a sandy concrete-like crust on them. Although I guess that would be very rare.