Okay, so! Following a brief chat with Five's player below — they'd like to keep cure options open at this point, so we'll have to be a bit minimalistic with the amount of info Fox can get, as we can't have him arriving at the solution.

So, for now, let's go with Fox being able to tell that there is now magic or a second (magical) presence linked to Five. He'll have to ask Five to ascertain whether this is a new development — unless he'd done a magical examination of Five prior to his absorbing the mirror energy, then it would stand out that this is a new situation. Fox can also sense that this magical entity/ability is deeply rooted in Five, and that it is passive/consuming, rather than generating.