Measurement technique Pokrywka invented for SXG: include a specific sub-resource that is sometimes loaded and use JavaScript to detect whether it was fetched from the prefetch cache. Because SXG prefetching is all-or-nothing, the presence or absence of the canary reveals whether the full set of sub-resources was prefetched successfully, and analytics can tag page views accordingly. He plans a future 'poisonous' variant that always fails, to inject for A/B testing specific pages.