I suppose that there are two ways to have determined that the size and texture of a Dugong is similar to that of a human, one would have been an examination performed by a marine biologist but the more obvious initial discovery would have been the same way that the mermaid myth originated, by direct experience!

Unfortunately, I can't remember exactly where I read about this or I would cite it but I assure you that despite having been in the region I am not speaking from personal experience.