The ability that governs bonus spells depends on what type of spellcaster your character is: Intelligence for wizards; Wisdom for clerics, druids, and rangers; and Charisma for bards, paladins, and sorcerers. In addition to having a high ability score, a spellcaster must be of a high enough...
•The user must have the requisite ability score. If the user meets all the requirements noted above, and her caster level is at least equal to the spell's caster level, she can automatically activate the spell without a check. If she meets all three requirements but her own caster lev...
Essentially, whenever a player attempts any action that might succeed or fail – assuming it’s not an attack roll or saving throw – the GM simply has to determine which ability score it falls under. No matter what it is, the GM just needs to say “Strength” or “Wisdom” or whatever...