I'd bet lots of telemetry, comprehensive design and change documentation, along with engineers tacit knowledge.

Something like:

telemetry shows dramatic drop of temperature on this, that given the location of the sensor could only be caused by a specific LOX line leak, and vibration sensors show data compatible with friction as the ignition event and not a short circuit because the relevant telemetry doesn't show any electrical abnormality, so, by exclusion, given no other anomalies, give that computer simulations show it is a feasible scenario, followed by lab work with a physical model, this must be the cause of the accident.