2d - How do I detect how much of an object has been "cleaned" by the player?

I want to let the player wipe some dust off of a book, then start a short animation.

Here's what it looks like:

I'm using a clipping region on the "dusty" book bitmap image to progressively hide the dust as the player mouses over it.

However, I'm stuck with how to detect when the dust is all gone. What's a good way of tracking how much is left?