def action
  u = User.new
  # do something...
  u.save
  cookies["user_id"] = u.id
  session["new_user"] = u.id
end

def action2
  u = User.find_by_id( cookies["user_id"] )

  if u.nil?
    user_id = session["new_user"]

    unless user_id.nil?
      # new user...
    end
  end
  # do something...
end