I have installed many grunt plugins so that I do not know do a plugin had been installed when I want use it. how to list installed grunt plugins.
grunt --help lists entry per installed plugin. Mentioned package.json is a place to see whether a project contains certain plugin.
Edit:
As you pointed, these are the plugins of certain project only. You wanted to know what plugins you installed to your machine.
If you installed them via npm, please use
npm list -g --depth=0
Source: http://ponderingdeveloper.com/2013/09/03/listing-globally-installed-npm-packages-and-version/