Let me prefix this answer with: This was based on the assumption the question was asking how to identify the version from the output CSS shown on the website. Which you can find here, which you will notice doesn't retain the version or license.
Unfortunately, joomla rip out the attribution from the CSS file referencing bootstrap so it would take quite a bit of investigating (seeing what changes had been made from 2.3.x to another version of 2.3.x) - however it is easy to be able to tell if its a 2.x.x branch or a 3.x.x branch.
Obvious way to check is just to look at the css, if it references items such as span1, span2 ,span*
etc, it's part of the 2.x.x branch.
If however you see references to col-md-4, col-md-5, col-md-*
then you can be sure it's of the 3.x.x branch.