This doesn't require any roll. Switching perception only takes a second and a Magician can still take full actions before or after switching to Astral or back to normal perception. While perceiving Astrally, all skill rolls involving interaction with the physical world suffer a -2 penalty. The Magic skills can be used to examine the Astral signatures of spells, spirits, talismans, etc.
While in Astral form, Charisma is used in place of Strength, Intelligence is used in place of Dexterity, and Wisdom is used in place of Constitution.
Fast Astral movement is 3+Wis mod x 1000km/hr.
This is a Perception check, modified by Wisdom. If you're tracking something passive like the signature of a spell, you get a +2 bonus for spells or effects of levels 4-6 and a +4 bonus for stuff that is levels 7-9. (It's hard to hide powerful mojo)
If you are tracking something that is evading you, like a spirit or an enemy Magician who is trying to cover up his Astral sig, they get an opposed Magic/Sorcery test to hide from you. Dual-Natured beings or spirits may substitute Stealth if that skill is higher.
Resolve as normal combat, but make sure to use Astral attribute equivalents as outlined above under Projection. Damaging or healing a character's Astral form affects hit points as per normal.