title: How to check if a gem is installed?
body: <p>I installed data_mapper for a Sinatra project. Curious, why is it when I do <code>gem install brew</code>, I can <code>$ which brew</code> and get the path of its location and can't for data_mapper? This works for some gems and doesn't for others. </p>
  
  <p>How do I verify a gem is installed properly? Would checking the version assure the gem is downloaded correctly?</p>