It's a PPR situation. You have $8, know that you're going to have $8 next turn, and you know that the last cards in your opponent's deck and current hand are a Gold, five Coppers and a Moat. If he has the Moat in his hand, he can draw all of his Treasures, meaning that he will be able to buy a Province, which means you have to buy a Duchy now, but if he doesn't, he only has 5 cards which can't buy a Province for him, which means you can safely ignore PPR. If Goko gives him the option to reveal the Moat after you've played Mercenary, you know that he has one in hand, and if not, you know he doesn't.