The fastest way to see the RDoc for your installed gems is to type the following command into a terminal, and point your Web browser to the address it returns (usually http://localhost:8808)
The Web server created by the gem server command contains the RDocs for all the gems you have installed, unless you disabled RDoc generation when you installed your gems.
I was used to either searching via Google, or going to a gem's RubyForge page to see the RDoc for the gem.
Initially, it seemed that the bit of effort is worth not having to learn some way to generate and bring up the RDocs myself. To my surprise, the procedure is very simple, and it's totally faster than browsing to some Web site that has the RDocs.
That aside, using a local server has the advantage that you'll see the RDocs corresponding to the exact versions of your gems. And, last but not least, RubyForge seems to be slowly falling into oblivion, and newer gems don't seem to bother publishing their RDocs.
I hope this post saves you some time.