Halas I think that it is not possible yet in Photoshop. everything that is in the "big data" as they call the surrounding area cannot be visible.
In fact, I've seen numerous times a features request for such a behaviour in Photoshop, we can only ask that it'll make to the next revision of Photoshop.
The only workaround I know is to select the move tool [V], then click on the "show bounding box" radiobox. This way, you'll know how far goes the off canvas data. (you'll nead to click on each channel to see its limit)
Not also that it will show only the 50% opaque areas.