Originally Posted by AnemicOak
To update them you'll need to re-download the books. I don't think there's a way to know for sure, but most books I've downloaded since K4PC started supporting KF8 came as a KF8 version (still a few older mobi's and some nasty Topaz).
Another way might be to download a sample (click the Product Description for each book to get to its product page and order a sample). KF8 has wider line spacing for some reason so it should look obviously different. No need to re-download if it is not KF8.