There is similar question here.
Ruby: Rails: Which version of a gem is used?
But it's about when
% gem list | grep selenium
selenium-webdriver (2.53.0, 2.48.1, 2.46.2, 2.45.0)
You probably have more than one gem because each project with Gemfile in your machine have one different version of it.
When using the gem via
require or command line without specifying the version the last one - the greater version - will be automatically used.
By convention, in the most part of the cases, you can print the version doing the following:
require 'some_gem' puts SomeGem::VERSION # => "3.0.3"