How do I find all installed packages that depend on a given package in NPM?

You’re looking for https://docs.npmjs.com/cli/ls

For example, to see which packages depend on contextify you can run:

npm ls contextify
app-name@0.0.1 /home/zorbash/some-project
└─┬ d3@3.3.6
  └─┬ jsdom@0.5.7
    └── contextify@0.1.15