Instructions
Play the game until you are in a battle with an enemy on whom you want to use the "Libra" spell.
Select "Libra" and then use the d-pad to select the enemy you want to target with it. Press the "A" button to confirm and your character will cast the spell. The words "Analyzed enemies weaknesses" will appear on the screen to indicate that it worked.
Select the "Attack" command and use the d-pad to highlight the creature on whom you cast Libra. Instead of pressing "A" to confirm the attack, look at the bottom screen and you will see the information revealed by the spell. The enemy's name, remaining health and weaknesses are all displayed here.