I am using npm scripts with watchify and node-sass with the -w parameter.
"watch-js":"watchify dev/index.js -o dist/bundle.js",
"watch-css": "node-sass -w my-styles.scss style.css",
"thewatcher": "npm run watch-js & npm run watch-css"
Yes, that is exactly what is going to happen. Script "thewatcher" will run "watch-js" and "watch-css" independently. Watchify will only listen to changes made to dev/index.js and node-sass will only listen to changes made to my-styles.scss. If you want to know exactly when a new dist/bundle.js file has been written, modify your "watch-js" script to include the -v flag:
"watch-js":"watchify dev/index.js -o dist/bundle.js -v"