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 typically looked on as an act of aggression.
Although guards cannot be called on your for snooping and nor will you be flagged as a criminal, it's use will lower your karma. You might consider the use of the Stealth skill as most players will not let you get close to them if they suspect ill intentions.
In order to peek, you must find a target with a backpack - For example, another character, or a Pack Horse. While standing no more then a single tile away double click the pack. If successful, you will be able to see the contents in the same way as your own.
Whether you succeed or not, there is a chance you will be noticed (nearby players will receive a message concerning your action), and if hidden, a chance of being revealed. Higher levels of skill increase your chances of success while decreasing the chances of detection.
Snooping is an essential skill for the budding thief in that once you can see into a pack you may target a specific item to Steal. Without the use of Snooping you can only blindly target the characters themselves and hope that the random item you grab will be of some value.
Player owned characters or pets can not be Snooped outside of the Felucca facet unless they are in the same Guild as you. Be warned, snooping your own guildmates without permission may lead to your swift and sudden eviction!
Find an NPC, bring up their paperdoll image, and double click their backpack. You might consider creating a single button macro for "last target", tap this approximately once per second until the desired skill level is reached.