When I want to know where an executable is, I use which like so:
$ which rails
/Users/jbranchaud/.gem/ruby/2.1.4/bin/rails
That is the rails binary on my path that will be used if I enter a rails command.
However, with something like rails, there may be multiple versions on your
path. If you want to know where all of them are, you can use where, like
so:
$ where rails
/Users/jbranchaud/.gem/ruby/2.1.4/bin/rails
/Users/jbranchaud/.rubies/2.1.4/bin/rails
/usr/bin/rails