I use Pagina Epubchecker, which is a Standalone application for Windows, Mac OS X and Linux. Homepage:
https://github.com/paginagmbh/EPUB-Checker
Download page for the binaries:
https://www.pagina.gmbh/produkte/epub-checker/
(top part of the page is in German, English is farther down the page)
Down-side of using this app is that you have to present a saved file, you can't do validations on the fly as you edit.