Other than going to http://search.npmjs.org and search for it?
I usually need the version to put it in my package.json.
package.json
Use npm view [package-name] dist-tags in the command line.
npm view [package-name] dist-tags
Example:
$ npm view express dist-tags { latest: '2.5.9', '3.0': '3.0.0beta2' }