In short, it cannot be easily determined whether the various plugins you have installed - out of the 23,499 available in the WordPress repo, as of this writing - are currently doing anything.
If a plugin is activated, it is "being used". Whether or not a given plugin is actually doing anything is an entirely different matter. Since plugins can do a near-infinite number of different things, it would be near-impossible for there to be a way test all past and future plugins to see if they are doing anything.
As an example, say you have a shortcode plugin that you use with
[some_shortcode]. You could search your posts for that shortcode, and if your search comes up empty, you know it's not being used. But other plugins do entirely different things, like caching content, adding SEO, enhancing Social-ness in your blog, changing your footer text, etc. How would you create a test to determine if they are "being used"?