Verify that a pixel is installed. Fetches the page named by the required ?url= parameter server-side and confirms the install script is present in its HTML, which verifies a fresh install before any visitor traffic exists, and counts the pixel's events over the last 24 hours; events are queryable within seconds of a page view. Either signal verifies. Tag-manager installs (for example Google Tag Manager) inject the script at runtime and are not detectable by the page scan; a page that cannot be fetched reports fetch_failed, which means "could not check", not "not installed". Name the target project with the ?project_id= query parameter.