Naturally, some guys are just built to be great runners, and people could tell from an early stage that they will be elite. (Ian Dobson and Mo come to mind.)
But I have heard a few stories about how certain coaches like Wetmore are very good at spotting guys who are very good HS but not great highschool runners, and then turning them into phenoms at the next level.
I have also heard about highschool coaches who are very good at spotting talent and can look at a bunch of freshman that all run the same time but can tell which ones have the greatest potential. The ones with the most potential may not be the tallest or fastest or most athletic-looking, but some coaches somehow know that a cetain kid is special.
My question is, basically, how can some coaches tell which runners can be truly great?