gistfile1.rb

class UserOption < Slop::Option
  def authenticated?
    user = User.find_by_username(value)
    user && user.authenticate(opts[:password])
  end
end

opts = Slop.parse do
  on :user=.     'Your user name', builder: UserOption
  on :password=, 'Your password'
end

if opts[:user].authenticated?
  # ....
end