There are not one but two unmarked turns on this hike. In fact, this is my second trip to Duke's Creek Falls. The first trip was a fail because I took a left instead of a right and followed a random trail to nowhere. Upon turning back I gave up and instead visited Raven Cliff Falls which is close by.
So here's the deal: Follow the trail until you come to a flight of downward stairs that terminates in a left / right choice. GO LEFT.
Then you will come to another T style intersection where you can see the post for the guide sign, but the sign itself is gone. Now this option is a little insidious as the left turn is a wide, soft trail that looks quite used. To the right, however, is a narrow, hard trail with a sharp, imposing dropoff to the left side. TURN RIGHT. Trust me, a left turn takes you further and further away from the falls. I lost almost half a mile going this way, so heads up.
I guess the old adage of waterfall hunting (that I just made up right now) is true here, if the sound of the water doesn't get louder, you're going the wrong way!