Item Identification
Choose Item Identification in the skills window then click on an item. If done properly, you will see the selected item's information and its properties.
How to train your skill:
Item Identification is the most difficult skill to train for Mage.
You can use pretty much any item to train your skill, but only when nobody has trained on the item you've chosen.
Advice: buy as many Crafters' items as you can, for example, part of weapon and armor they don't need anymore. Also, you can buy some items from vendors (e.g. runes), use your skill on the items and sell them again.