This will be one of the first trophies you get in the game. In the dungeons, you will sometimes come across extra-large treasure chests. Usually, these are locked. You will be given the choice to use a Treasure Key or Force the Chest Open. Choose to force it open. You will then be given a chance to guess which trap the chest contains - poison, explosion, or curse. I always pick poison (though it doesn't really matter). If you get it right, the chest immediately opens and the trophy fires. If you get it wrong, you get the answer because the trap in that chest won't change. Just choose whichever hit you, and the trophy is yours. On a side note, I would recommend never using Treasure Keys. They sell for 400 gold each, which is a very nice price in this game.
On a side note, you can catch Mardan Garayans (magic fish) at the Waterfall in Matataki Village. You must fish at dusk with poison apples. Only Mardans will bite a poison apple, so you just keep trying until you find a fish that wants it (usually a larger fish). Once you have caught a Mardan, it will always tell you which trap is in the chest. How this might affect the trophy if you don't have it yet, I don't know.