To check if a specific package is installed globally execute:
npm list -g [package-name]
Let's take "grunt" as an example.
If it is installed globally, you should see something like this
C:\data\tryout\test1>npm list -g grunt
C:\Users\xxxxxxx\AppData\Roaming\npm
└── [email protected]
If it is not installed globally, you should see something like this
C:\data\tryout\test1>npm list -g grunt
C:\Users\xxxxxxx\AppData\Roaming\npm
└── (empty)
To check if a specific package is installed locally you can execute the same commands as above but without the -g parameter.
source: How to check if npm package was installed globally or locally.