How can I check if an ePub ebook was fully and correctly downloaded onto my iPad or iPhone without being corrupted? I want to be sure to read it in those situations where I can't connect to the internet, like on a plane.
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.
Ereaders like Kindles, Kobos and so on are devices that can connect to internet to sync their contents and to provide some form of social networking capabilities, but otherwise are meant to be used with local ebook files. What you call "offline viewing" is really the only way to read ebooks on these devices.
Epub books are basically a particular type of a
.zip file; its contents are made of many different html files, there are no single "pages" with the same meaning of a paper book.
As a rule of thumb, if there were problems during the download that corrupted the file, you should not be able to open it with your ereading software at all, or get some errors. On the other hand, if you can correctly open it and read its contents, you can be reasonably sure that the file integrity is all right.
Another test that you can easily perform, is to simply browse your ebook and directly see if all of its parts are displaying correctly; a properly created ebook has a table of contents where you can check, .i.e., the supposed number of chapters, and try to navigate to each one of them to see if it is available/accessible. Easy as it sounds, a quick check like this is maybe the best way to check if the file is not damaged: if everything seems all right, well, everything is all right.
Anyway, you should also consider that usually, I dare say on the 99% of the cases, an ebook file size is just a couple of megabytes or so. It is very unlikely that with this really small size and consequently short download times, there could be such problems on the network that could damage your file; even in the worst of the hypothesis, redownloading the ebook is just a matter of seconds.
Summarizing, your concerns are perfectly understandable, but in a real world scenario, they are likely to happen in the 0,01% range, maybe.