Short answer is right now you really can't.
We use a third party solution - Dropout Detective - but that's overkill for what you're looking for (we use it for our online students to track those who are at risk). I'm guessing if you have someone who is good at digging into the back end of Canvas you could program something to pull this information. Something else to think about is that Canvas is coming out with hosted data services (possibly later this year?) that might allow you to get this information.