Use the skill, then click on the item to be identified. If successful, information about the item will be displayed.
Raising Item Identification: Item Identification is an Activated Skill on the standard 10 second cooldown. Item Identification is not a difficulty based skill. You can repeatedly ID the same item until 100 skill.
Gideon's Advice: Having a GM Item ID character is incredibly useful as you can identify many items at one time. Target any container, corpse, treasure chest and all of the items in the root container will be identified.