About the Skill:
Curiosity killed the cat, and in Britannia it can certainly put you in hot water.
By and large, Snooping is considered a prerequsite for the Stealing skill. A character only can steal random items from other characters, NPCs, or pack animals unless they possess the Snooping skill. Like the Hiding skill, Snooping can be practiced to its own end, i.e. just to be able to see what others are toting around Sosaria. However unlike Hiding, it is typically looked upon as aggressive act.
You cannot be Guard Wacked for Snooping nor will you be flagged as a criminal. Snooping 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.
There are no Power Scrolls for Snooping; the maximum skill for all players is Grand Master, or 100 points. And while there is only one item that has any Snooping skill points (i.e. Burglar's Bandana), there is no cool down timer for this skill's use, so the gains come that much quicker.
Snooping only allows you to see into other packs:
the backpack of another character
Pack Llamas; and
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 you see your own backpack.
Whether you succeed or not, there is a chance you will be noticed, wherein nearby players will receive a message concerning your action, and if hidden, a chance of being revealed. Your chances of success increase with higher levels of the Snooping skill, while your chances of being detected decreases.
Snooping is an essential skill for the aspiring 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!
How to train:
Visit a bank or moongate with a high amount of traffic. Locate a pack animal; unattended pack animals are the best as they don't move away. Simply double click the animal. Alternately, you can find an NPC that is outside of a guard zone, bring up their paperdoll image, and double click their backpack.
Regardless of your target, you might consider creating a single button macro for "last target", tap this approximately once per second until the desired skill level is reached.
Both Alacrity and Transcendence scrolls feature the Snooping skill.