active_model_serializers/docs/howto/test.md

How to test

Test helpers

ActiveModelSerializers provides a assert_serializer method to be used on your controller tests to assert that a specific serializer was used.

class PostsControllerTest < ActionController::TestCase
  test "should render post serializer" do
    get :index
    assert_serializer "PostSerializer"
  end
end

See ActiveModelSerializers::Test::Serializer for more examples and documentation.