A target of most mana spells will never know who hit them with it unless there's something special about the situation, like the target also being a mage who happens to make a perception test to see it out of the corner of his/her eye, the casting mage has a geas that requires V/S components when casting (haha, D&D terminology, but still applicable

), etc. (Yes, there are rules for handling noticing a spell being cast. I'm talking about noticing it otherwise.)
And as far as it being an advantage over automatic weapons, it would depend on the number of targets. Only one? He's not going to know that it was an automatic weapon any more than he will have known it was a mana spell. He'll be dead before the report reaches him. (Assuming one-attack kills in either case.)
And if the mana spell doesn't kill you, yeah, you'll know that you feel like utter crap and be in pain all the sudden, but only someone familiar with magic and spells is likely to automatically realize it was a spell, and even then they won't know where it came from unless they've figured it out through other means.