[Bradd]

You can establish line of sight for a spell "using reflective surfaces and through transparent objects" (p. 183, SR4A). Spell resistance tests include "any positive cover modifier the target might benefit from" (same page). I'm wondering how to apply cover if you target somebody through glass or in a mirror.

When ... the defender’s form is obscured by intervening terrain or other forms of cover such as brush, foliage, or various obstacles (crates, windows, doorways, curtains and the like) he benefits from a ... modifier to his Defense test. Note that this modifier applies to all Ranged Combat tests and also to Spellcasting using line of sight.

If my spell target is behind a pane of glass, is he "obscured"? My gut feeling is that clear glass should offer no cover from spellcasting. Tinted glass would either give cover or create a Visibility modifier, not sure which.

If I can only see my target in a mirror, is he "obscured"? In this case, I'm tempted to say at least partially yes, but I'm not confident about the ruling.

[Chaemera]

Here's your catch:

As they travel down the link to the chosen target such effects may be impeded by physical obstacles or mana barriers. They may impact transparent obstacles (such as glass) and do not "bounce" off reflective surfaces used for line of sight.

Indirect combat spells are the only spells treated as ranged combat, so it's the only time you'd have to worry about cover/obscurity from using a mirror or shooting through glass. However, they stop at either.

[Bradd]

Visibility and cover modifiers always apply to spellcasting, not just for indirect combat spells. See the text I quoted on p. 183.

[Kontact]

Well, if you can see the target in the mirror, imagine it like seeing the target in a doorway.

Shooting through a window with a spell shouldn't be much different than with a gun.  50% cover, 75% cover etc.. 

[Chaemera]

Visibility and cover modifiers always apply to spellcasting, not just for indirect combat spells. See the text I quoted on p. 183.

Fair enough.

If you can see him in the mirror, unless it's one heck of a weird mirror, then visibility would be the same as ambient light.

For cover, I would track LOS from caster, to mirror, mirror to person, and determine what, if any, cover intervenes and use that. The mirror itself, unless it were particularly small, I would disregard, though Kontact's doorway/window analogy is also a valid route to take.

[Bradd]

The doorway analogy matches my intuition too. That's actually why I was leaning toward using cover most of the time: Unless the target fits fully within the frame of the window or mirror, it would have at least partial cover.