The evaluating intelligence skill must be use to “assess” a target creature.
Chance for successfully assess the target is:

• perk level * 5%

When successfully assessed, an overhead message indicates wether the creature is susceptible to enery or fire spells.
Every player that has the Target Assessment perk active and uses a spell of the respectively category on the target creature, gets a damage bonus.
Damage bonus: 10% per perk level

Supported Spells

• Fire Spells: Fireball, Explosion, Flamestrike, Meteor Swarm
  
• Energy Spells: Lightning, Energy Bolt, Chain Lightning

Increased the effectiveness of Greater Heal and Heal Spells: Bonus: 10% per level

Increases spell damage on target creatures that are below 50% of their health.
Damage bonus: 10% per level

Adds 300% magical damage (to the spell that triggered the proc)