Use the skill, then click on the character you wish to evaluate. If successful, text will be displayed describing his mental capabilities.

Raising Evaluating Intelligence: Evaluating Intelligence is an Activated Skill on the standard 10 second cooldown. Evaluating Intelligence is not a difficulty based skill. Use Evaluating Intelligence on mobiles (players, monsters, animals, etc.) in the world until you reach 100 skill. Evaluating Intelligence is also gained when casting offensive spells.

Gideon's Advice: Evaluating Intelligence actually gives fairly decent Intelligence gains and is pretty much a must on all mage characters. Keep in mind that you can combine it with Anatomy and the two will act as passive Wrestling skill. It also increases while casting spells so you don't have to macro it all the way to GM if you plan on also doing Magery.