Вы можете использовать validates_confirmation_of в вашей модели.Из API:
Model:
class Person < ActiveRecord::Base
validates_confirmation_of :user_name, :password
validates_confirmation_of :email_address,
:message => "should match confirmation"
end
View:
<%= password_field "person", "password" %>
<%= password_field "person", "password_confirmation" %>