It would be helpful to have some type of indicator in both the Timeline and Storyboard views to indicate if there is a mask associated with any particular layer.
Currently the only way to know if there is a mask is to click on any layer and visually see if the Edit (Mask/Layer) button pops up next to the Scale wheel (or appears in the options palette on the right).
In the Storyboard view, perhaps a small icon placed along the top of the layer image (in the border next to where the “!” indicator is currently placed)? In the Timeline view, perhaps a small icon placed directly on the layer image (color might be a problem in this case). Or again, perhaps next to where the little “!” icon is currently placed.
Just anywhere the user can get a quick visual notification that a mask is being applied to a specific layer. Right now it’s guesswork, especially if you have a large slide show and forget whether a mask is being used or not.