Snooping allows thieves to attempt to open the inventory of other players, pack animals and NPCs.
Open the Skill Book (press K in-game) and browse to the Snooping skill, drag the Snooping icon to your hotbar and use that button to activate Snooping; additionally, beings that are in-range and can be snooped will have a right-click option of "Snoop"
You must be within a step or two of the target in order to Snoop them.
Snooping can only be performed against characters with backpacks:
0-40 Train from the NPC Thief Guildmaster in Britain
40-100 Use Skill on a stationary target
Consult the current Rules for allowable use of macro utilities.