If in a recent version of Sigil, it will tell you if your epub is an epub3 or epub2 by looking at the main gui Window name.
The definitive way is to look at the opf file. The content.opf file has an opf package tag that has a version attribute that determines the version of the epub:
For example, here is the opening package tag for a typical epub3 content.opf.
Code:
<package xmlns="http://www.idpf.org/2007/opf" version="3.0" xml:lang="en" unique-identifier="uid" prefix="rendition: http://www.idpf.org/vocab/rendition/# cc: http://creativecommons.org/ns#">
Notice the version="3.0"