@jose this can be very dependant on the plugin in question as they can be used for a very wide range of solutions. One approach would be to look at the plugin readme, often they provide a macro or widget and you can search your tiddlers for invocation of that macro or widget, if found you can stop searching and keep the plugin, if not found you can dig a little deeper once you understand what functions the plugin delivers.
- Perhaps there will never be a general rule for this.
Disabling plugin, save and reload then using your wiki for a while to see if the lack of the plugin has an impact is another approach, is a few weeks you can delete all disabled plugins, however I recommend doing the above first before relying on this approach.