Draw a vertical timeline per worker/stream and trace each action. When event-sourced reactions run as separate handlers (e.g. a 'free gift' policy reacting to 'item added' events on another stream while the cart command continues), the timeline forks. The diagram does not prove a race condition — it tells you that you must analyze for one. Tests run the system; this technique analyzes the system, which is why it catches what testing misses.