Just asked support about this -- was told they don't offer it this feature and that I should copy the list of opens and compare it to the main list. But the opened list is a dynamically loading window while you scroll (and automatically closes when you get to the end), making it (so far) impossible to copy a list of around 2,000 subscribers.
Very annoying -- why do they not want us to target our most engaged subscribers? Seems like a no-brainer of a feature for a marketing product.