So this came up in a recent session, and I have no idea how to answer this problem or where to start looking. We had a warlock, and we were fighting a custom monster that was capable of shapeshifting into the form of other PC's. It would grab a PC, then the monster and the PC would teleport, and when they reappeared POOF! Two PC's. You know, to confuse them and maybe make them hit each other if they have bad perception rolls.
The question was this: can the Warlock tell which is the PC and which is the shape-shifted monster if the monster had been cursed? If not, how does the Warlock's curse work if the player has no knowledge of whether the defending creature is the cursed creature or not?
Similarly, how does the curse operate when a creature turns invisible? I assume the Warlock can't use the curse to locate an invisible creature...
Sorry if this question has already been asked, but it seemed like the kind of thing that doesn't come up often.