shoulda is a library build on Test::Unit that provides helpers, macros and assertions to make testing easier.

rcov is a code coverage tool for Ruby.

1. Install rcov

sudo gem install rcov

2. Install shoulda

sudo gem install thoughtbot-shoulda --source=http://gems.github.com

3. Create your Rails project

rails myapp

4. Modify myapp/Rakefile

require(File.join(File.dirname(__FILE__), 'config', 'boot'))
require 'rake'
require 'rake/testtask'
require 'rake/rdoctask'
require 'tasks/rails'
require 'shoulda/tasks'
namespace :test do
  desc 'Measures test coverage'
  task :coverage do
    rm_f "coverage"
    rm_f "coverage.data"
    rcov = "rcov -Itest --rails --aggregate coverage.data -T -x \" rubygems/*,/Library/Ruby/Site/*,gems/*,rcov*\""
    system("#{rcov} --no-html test/unit/*_test.rb test/unit/helpers/*_test.rb")
    system("#{rcov} --no-html test/functional/*_test.rb")
    system("#{rcov} --html test/integration/*_test.rb")
    system("open coverage/index.html") if PLATFORM['darwin']
  end
end

5. Modify myapp/test/test_helper.rb

...
# Add the following line
require 'shoulda/rails'   # require 'shoulda' also worked
...

Conclusion
After you’ve written some shoulda tests, you should be able to use the following rake commands:

rake test
rake test:units
rake shoulda:list    # display specs from shoulda tests
rake test:coverage   # run rcov and display code coverage