Reading the story feat Twisted Love made me think about this, because it say:
You retain your +2 bonus on saving throws against enchantment effects. If you succeed at a Will save against such an effect, the caster does not learn that you succeeded at your saving throw (effects such as detect magic and analyze dweomer still work normally).
For me this implies that a caster of an enchantment spell knows when her spell failed to work on a target, but I do not remember seeing this anywhere else in the rules (and in my games, it was usually used as a house rule for some spells like Zone of Truth). So my question; does the caster of an enchantment spell always know if the spell worked or failed?