There are two main validation tools. One it EpubCheck and the other is Flightcrew. Flightcrew is also built-in into Sigil.
In general Flightcrew/Sigil will give you better understandable error messages.
Be aware though. They will check whether your ePUB is structurally correct. That does not always mean that your ePUB will render as you would like. That depends on the reader application.
Another tools which is quite valuable, is the
CSS validation tool. An error in the stylesheet will often result in the reader application completely ignoring the stylesheet.