NOTE: The following covers how AR is/can be handled in the real world since I can't freaking remember where my SR4 book is at the moment and I don't have an SR5 book.QUOTE (Cain @ Sep 30 2013, 03:00 AM)

This is how I've always ruled it. You're free to disagree, but this has always worked for my tables.
In many spots, it refers to AR as an "overlay". So, it's laid over your normal vision. In other words, it modifies what you see, it doesn't replace it. Therefore, you can target through an AR image.
Being an overlay actually doesn't pertain to the question of if spells can be cast while using an AR device.
How AR is handled with said device is the key. Generally speaking there's two ways of doing AR, I'll call them the Digital and Optical methods for lack of knowing any better terminology. These terms are also useful since they're similar to magnification and how that works with casting as well.
Digital Method - Common Usage: Goggles
This method works via a camera/screen system with the camera(s) replacing the user's eyes and sending the feed, modified by AR, to the screens which are then viewed by the user. Natural light is not obtained by the user and thus, spells can not be targeted while wearing the system.
Visual:
Light>---Camera>---AR>---Screen>---Eye
Optical Method - Common Usage: Glasses
While similar to the digital method, this way instead uses translucent screen(s) where the
only output is the AR overlay. Natural light passes through the screen which applies the AR overlay(s), and is then naturally perceived by the user thus, allows for spell targeting.
Visual:
Light>---Camera>---AR>---Lens>---Eye
&
Light>--
Lens>---Eye
---
Example: Simple room, empty save for two targets. Target A is real, Target B exists only in AR. Two mages walk into the room and their AR device puts an arrow on Target A to denote that it is the real target.
Mage A is wearing a digital AR device, and while able to tell that Target A is real, is still unable to cast a spell since he's viewing the world through screens.
Mage B is wearing an optical AR device, notes that Target A is real and zaps it with a powerbolt.
---
While the above merely covers the method of AR display that covers both eyes, there's also single-eye variants where the device only covers one eye(Google Glass is a good example of this. The Eye Drives from Doctor Who could be considered similar as well). In that case, Mage A would also be able to zap the target. If AR was processed through cybereyes, there shouldn't be any issues there either IIRC.
In short, it pretty much comes down to how AR is displayed by devices and whether or not the overlay covers one or both eyes. Which I realize was said rather early on in the thread. However, digital setups would have a larger range of extras that could be integrated into the device such as thermal, image mag and the like.
Extra Fun:
Due to how I understand SR5 is doing things....
Digital AR devices could be bricked and impair vision in the eye(s) the display covers potentially requiring that they are removed before the user can see clearly again. Optical devices, if bricked, I'd rule that they wouldn't obscure vision but merely lose AR functionality while both are subject to being hacked to mess with the AR overall(adding/removing overlays for instance).