Ideally, all active plugins should provide an “Am I being used?” option (alongside the usual “Deactivate” and “Edit” links on the WP plugins page). This would scan the system and detect whether or not there’s any current content, widgets, etc. that depend on the plugin being activated.
This might not always be 100% testable. In such cases, part of the test should include alerts regarding ways in which the plugin might be in use that are only manually detectable.
However, this kind of mechanical detection would help a lot when you’re scanning the list of plugins on a site, wondering whether or not all the active plugins are actually needed. Encouraging this kind of functionality might go a long way to helping reduce needless plugin overheads on WP installations.