I don't think there is a definitive answer to this question.
However, we do know this:
1) Gbot downloads your website's code and analyzes it
2) Platforms like WordPress, Joomla, etc., and their themes/plugins, all leave footprints in your code. So it's easy for someone to match those up.
3) However, at the same time, millions of people use the same platform and plugins, so it's really difficult and unfair for G to single out any such one
In the end, I think they could easily analyze and obtain this information but there's just not a practical or fair way for them to utilize it in ranking factors.