title_body: How to tell if an entire ePub ebook was downloaded for offline viewing?
upvoted_answer: I don't have a recent ipad and I rarely buy things through the iTunes store, so my answer is based on knowledge and guesswork. 
  
  When you first try to read it, the reading system (in this case, iBooks) will download the complete ebook if it has not done so already. 
  
  There is a simple way to test this. Open the ebook, and then put your device in airport mode (ie. no network or wifi). You can quickly check that you can get to the last page. I know, some itunes ebooks have multimedia content; that is a different story. In some cases, the multimedia is embedded and available only online (i.e., youtube). In some cases, it is actually part of the ebook, so it would fully be available in offline mode. 
  
  When I have downloaded purchased items from Google and Amazon, occasionally the download will be interrupted, but usually the cover icon won't appear on the bookshelf (or it may only partially appear). I assume iTunes does something similar.