If there's rot in a tiller, it'll probably happen at the point of attachment. I haven't seen your tiller, but most are vulnerable where they have holes drilled through the end for their attachment to a metal fitting. Water gets past the metal and into the holes, and the rot begins.
I snapped the tiller on my first boat, a 16ft. Grumman Flyer. I've always inspected the tillers on any of my subsequent tiller steered boats by simply removing the tiller and inspecting the inside of the holes with some sort of pick. Obviously, it should all be solid.
I replaced the tiller on my first keelboat, a 26ft Grampian. Inspection had revealed the end of the tiller to be iffy. I was too lazy to laminate a new tiller for it, so I bought a piece of ash, about 2" thick, 8" wide, and 5ft. long. I traced my desired tiller on it, cut it out with a jigsaw, rounded the corners with a router, and did the inletting for the metal hardware with a chisel. I drilled the necessary holes, and then coated the holes and the entire tiller with penetrating epoxy followed by several coats of varnish.