This is somewhat of a simple question but does have important consequences, can a magus fast-cast a response/defense/unraveling spell if the attacker is using a sight range spell at such a distance that magus has no way of identifying the tech/form or even the intent or target?
This question does assume that the wizard is aware that the distant maga is attacking them.
Yes. In my not-very-humble opinion.
I don't know of any reason why he shouldn't be able to do so. And you need to consider the consequences of saying 'no, he can't'. The can of worms that would open is very large and smelly. Do you need me to go into them?
I would make the ease factor to identify the incoming TeFo higher than normal* if the defending mage can't see the gestures/hear the words, but he/she still gets the roll.
And there are defenses that do not depend on knowing what the incoming spell is eg a teleport spell, or PeVi general.
*Noble's Parma: I haven't checked to see if there is a list of ease factors for this already published.
I think the rules for this are sufficient: automatic detection of Form, unless the gestures are subtle. Other times, the magus must make an awareness + per roll exceeding 15 - magnitude of the effect to determine the form. I think that's generally good enough.
If the defending magus is aware of the attack, then yes, I think he can Fast Cast against it.
The RAW says that if you see what the opposing magus is doing, then you determine the attacking Tech. and Form automatically. Otherwise you need to make a roll.
So, if the attacking magus is too distant to clearly see what he is doing, then the defending magus will need to make the Perception + Awareness roll indicated in RAW. Depending on the circumstances the storyguide could also apply a modifier to the Ease Factor.
So in failing the Awareness roll to see accurately the distant attacking magus, then in your opinions, would the defending magus be incapable of unraveling a spell directed at them, or creating a defensive spell that opposes the form? Of course the Re/Co sidestepping spells and such would still work, but barring the obvious, a fast casted response would then be simply taking the chance that your defensive spell is going to be as broad a protector as possible to prevent harm. You could not simply undo a spell that you are not capable of recognizing.
Automatic identification is for hermetic magic however you do have to see it cast. Subtle/silent spells, spells too far away to recognize gestures or hear voice will require perception rolls to detect or might require the magic theory roll.
You might not be able to get a form specific spell to counter it but you can get a general counter magic spell off (like wind of mundane silence).
This is why a Wind of Mundane silence "to hell with it" enchanted item is a must have for most magi in our sagas. When you know you are about to be hit by something nasty but do not know what it is (and you cannot teleport away as a defence) you just nuke the magic from around you and learn to deal with the consequences of your magical ddisguise and magical wards dropping fast.
If he is aware of the attack (somehow), but fails the Perception + Awareness roll to determine the Form, then he can still Fast Cast a defence (if he makes the Speed roll). He just doesn't know the Form of the incoming magic. Which is fine, he will just either need to cast a broad defence that works against any Form, or guess the Form (if he knows that the attacker is Mad Roger the Flambeau, then "Ignem" might be a good guess, for example).