If your the target of a spell and you don't accept it voluntarily, there is a POW vs POW roll, so you automatically know you are 'under magical attack' because your soul is resisting. If it fails to be cast, logically, you don't know, but if it is cast successfully but don't manage to overcome your magical defenses or your POW, yes, you should know.
Yes, being out of sight and out of hearing range.
For me, and as we play it, the target of a spell automatically knows a spell is cast and by whom. Witnesses may know a spell is cast, depending on situation (see above), but would not know who is the target, unless it is obvious, like a sword bursting in flames, somebody dropping dead or the like.