There isn't a definate way to be 100% sure that a test flight has been done without being an eyewitness. When I do an event I announce a beginning and close so test flights or other types of flying are open. Not even a small problem with a flight before the advertised time.
The major repair comment is however an interesting one, particularly in say a combat or racing event, what dictates a major repair? I need to think about that one since most of the event's I've done are high speed and racing venues, ergo interpretation.