I think (and have played it this way for years) that if an attempt to penetrate your Parma Magica fails you are aware your Parma resisted a spell. Is this actually correct? I don't have my books with me tonight, and it would be useful t check if i was correct on this. Sorry to be a pain!
Yes. Main rule book, p. 85, under "Awareness of Attack."
"If a spell is stopped by magic resistance, the target knows that some effect has been stopped, but knows nothing about it. If a spell penetrates her magic resistance,she gets no warning other than any obvious effects the spell has."
So, if a spell is successful but is blocked by your Parma, than you know that your Parma has stopped a spell. If a spell is not successfully cast on you, or it is cast and its penetration exceeds your Parma, than you likely don't have a clue.
Or, alternatively, you have a really big clue, like suddenly being a pig.