Go to each dungeon. If you have the compass in it, see if the triforce room is glowing or just faded. If it's faded, just leave and go to the next. If you don't have the compass, get it or go to the triforce room.
Actually, which piece is missing? Each triforce has a specific place on that subscreen. Surely someone or something has it mapped.
Top left- Level 1 Top right- Level 2 Bottom far left- Level 3 Bottom far right- Level 4 Top left of middle- Level 5 Bottom left of middle- Level 6 Top right of middle- Level 7 Bottom right of middle- Level 8
Well, a surefire way to find out is to enter Level 9 and see if you can pass the second room. If the doors open, then the triforce is complete. If not, then you're missing at least one of the eight triforce shards.
"Er...well, y'know. You can't make an omelette without um...destroying a forest.....or something" -Black Mage
I used to always think about that while playing this game. If I was running around fighting monsters with magic shards in my tunic, I would drop one for sure. Or I would fall on it and get stabbed in the leg by triforce shard.