Mostly, it would depend on the type of spell. For some spell types, if the RR is successful, the person realizes a spell was attempted. If the RR fails, they may or may not know that a spell was cast, dependent on the spell type. If it is a mental control spell, such as sleep, charm, etc., they will not realize a spell is cast. If it is a spell that gives the caster physical, but not mental, control, then they do know the spell was cast and they failed to resist.
Other spell types would depend upon the GM.
rmfr