Thinking 'in universe' the obvious test would be to have a pegasus fly above her unobserved with objects and drop them, with another compatriot on the ground observing the timing of her various twitches. Having done this one could vary the situation to rule out various causes, for example telling the pegasus to drop it at a certain time, or at a random time, to see if the precognition is genuine reverse causation or extremely accurate subconscious predicition (unlikely, but they are magical so it is semi plausible).

Assuming genuine reverse causation was possible, one might be able to send messages back in time. For example if this coin lands heads I will drop the object. Though this may lead to paradoxes if the message arrives before the event causing it, and somehow changes the situation such that the causing event doesn't hapen.