Curiosity killed the cat, and in Britannia it can certainly put you in hot water. To assess their marks, thieves and brigands often embrace the Snooping skill. But beware — Snooping is often looked on as an act of aggression.
Purchase up to 100 pouches, bags, and backpacks from a Provisioner. Arrange them neatly in multiple backpacks so they are easy to access. Have a friend hold them in their own backpack. If your friend is in the same Guild, you can train in Trammel. Otherwise you will need to train on the Felucca facet.
Open your friend's backpack and then the first container holding all the to be snooped containers. Then simply go down each row attempting to snoop each container. You only have to snoop each container once. If you do not gain on one container you can not gain on that one until you gain on another. Once you reach a high level of snooping it is possible you will go through every container and not have a gain. You should then go and buy some more containers and have your friend switch between them until you gain again.