@GraciousMe: I suggest you to install Lucifox add-on for Firefox, and open your EPUB there. It is a quite picky EPUB reader, so you will get an extra "validation" round, plus the possibility of testing it in a resizable window. (The only notable limitation is that it does not support SVG images.) A similar test can be done with the calibre ebook viewer --- but I prefer Lucifox.
Unfortunately, there are still many things that would require testing on a physical device. Here in Italy we have founded a nonprofit organization, the
eBook Club Italia that, among other wonderful initiatives, offers help to authors by testing an eBook on several different eReaders, owned by members or by the association itself.
If you need help, you can contact me with a private message.