how to test params passed into a controller in rails 3, using rspec?

get :show, {:id => subcategory.id.to_s, :params => {:sort => 'title'}}

Should be

get :show, :id => subcategory.id.to_s, :sort => 'title'

Unless you mean to pass params[:params][:sort].

Also

helper.params[:sort].should_not be_nil

Should be

controller.params[:sort].should_not be_nil
controller.params[:sort].should eql 'title'

(If you mean to test a helper, you should write a helper spec.)