You can run npm list -g to see where global libraries are installed. On Unix systems they are normally placed in /usr/local/lib/node or /usr/local/lib/node_modules when installed globally. If you set the NODE_PATH environment variable to this path, the modules can be found by node.
Non-global libraries are installed the node_modules sub folder in the folder you are currently in. You can run npm list to see the installed non-global libraries for your current location.
Answer copied from this Stackoverflow question