Crossover Stats

Disciplines that use the target's Humanity as the difficulty:
On Mages: 1/2 Arete + 1/2 Willpower
On Werewolves: 10 - Rage
On Hunters: 10 - permanent Conviction

Gifts that target Rage as the difficulty:
Vampires: 10 - Path rating/Humanity (the higher the Beast, the greater the Rage).
Mages: 6 - Spirit

Gifts that target Gnosis as the difficulty:
Mages: Use their Arete. Ho ho.
Vampires: 8 or 9 (based off of Banality rules in Changeling).

How is magick perceived?
When Magick is /in use/ then naturally it's going to be easier to detect. Coincidental effects have a DC of 8-10. Non-mages will not know what the hell they sensed, nor who is the source unless it's obvious, but they can definitely sense something is off.
Vampires: Perception + Alertness (variable difficulty). Auspex 2 will let the user know as a dead giveaway.
Werewolves: Perception + Alertness (variable difficulty).
Mages: Perception + Awareness (variable difficulty - Awareness). Awareness 3 allows the user to immediately know.
True Magick: All supernaturals have a chance to detect Magick use.

How are other things perceived? Thaumaturgy, Hedge Magic, Disciplines, Gifts…
Mages make a Perception + Awareness roll. Non-supernaturals have no context to base any judgement off of them and can't roll this.

Vampires vs. Mages
Thaumaturgy is directly Countermagicked (diff. 7).
Dominate, Dementation, Presence, and other supernatural means of mind tampering are Sphere Countermagicked with Mind (diff. 8). Only attacks on the Mage himself can be countered.

Werewolves vs. Mages
Spirit Sphere Countermagick works against all Garou Gifts (diff. 8).

Witnesses and Magick
Vampires, Werewolves, Mages, Wraiths, Changelings, and all supernatural creatures are all counted as awakened, and are therefore not witnesses. Ghouls can be either awakened or be witnesses. It depends on whether they are aware they are Ghouls and have consciously honed their Disciplines, in which case, they're not a witness. Likewise, Kinfolk who have seen it all with a Garou is not so inclined to go bazonkers because he's seen crazy shit before.

Magickal Damage
All Spheres affect supernatural creatures just as they would sleepers, with the following exceptions:
Vampires: A Mage needs Matter 2 and at least Life 1 in order to affect a vampire. All other Spheres function the same.