in my project, I have something like this:

 63       before do
 64         user.projects << project
 66       end
 67
 68       it 'can create new task using valid attributes (creating new task)' do
 69         lambda do
 70           post :create, :project_id => project,
 71                         :task => valid_attributes
 72         end.should change(Task, :count).by(1)
 73       end

so for your case I'd try something  like:

    let!(user) { FactoryGirl.create :user }
    let!(task) { FactoryGirl.create :task, :user => @user }
    let!(valid_attributes_for_check_for_task) { FactoryGirl.attributes_for :check, :task_id => @task }

before :each do
    sign_in(user)
  end

   it 'saves the new Check to the database' do
      lambda do
        post :create, task_id => @task.id,
                      check => valid_attributes_for_check_for_task
      end.should change(Check, :count).by(1)
    end