If you find a man made hole with a perfectly vertical shaft and high aspect ratio (tall and narrow), it was drilled. Individuals can float or be washed ashore on an island, populations can't. If you find entire civilizations on distant islands, they got there by some sort of boat or advanced raft. Rope generally implies twisted or braided fibers, so maybe it's difficult to tell if this was artificially twisted or a natural one like a vine. But if it looked like a rope, and was used like a rope then it was a rope.