I found the output contained the information I was looking for:
$> npm install -g karma
> email@example.com install /usr/local/share/npm/lib/node_modules/karma/node_modules/socket.io/node_modules/socket.io-client/node_modules/ws
> (node-gyp rebuild 2> builderror.log) || (exit 0)
$> ls /usr/local/share/npm/bin
After adding /usr/local/share/npm/bin to the export PATH line in my .bash_profile, saving it, and sourceing it, I was able to run
$> karma --help
Hope it helps!!