Well, there is no way of knowing for sure.
It's not like you open up a chest and see your roll confirmation at the top of that chest "barter" window and then some additional information that the initial result was upped with 15.
And as long as that info isn't there for the entire group to see, the stealing of items from chests will continue.
I used to be with it, but then they changed what it was, now what's it is weird and scary to me.
Khyber: Baeylan, various Annarras