good coaches can evaluate where a runner's talents will come to fruition. by watching them run at various paces, the experienced eye can determine where a runner exhibits good economy. also, you can arrive at reliable generalities about body-type and race-fit. for example, two 18 year old runners you coach run about an 11 flat 100m. one is 5-11 140lbs, and the other is 5-11 160lbs. next, they both run 49 seconds in the 400m, but you notice some differences: the heavier guy with thicker muscles sprints 400m with higher knee lift, a shorter stride angle, more vertical displacement, and slightly faster stride-rate. the thinner guy sprints 400m with lower knee lift, a longer stride angle, a long back kick, virtually no vertical displacement, and a slightly slower stride-rate. at this point, you see that the 160lb guy is a fairly conventional sprinter who will almost certainly struggle running the 800m anywhere near 1:50.
you see the efficiency of the thinner guy and immediately know there's potential for sub 1:50 800m and, better yet, potential for a sub 4:00 miler. after watching him run 400's in the mid to high :50s during training, he exhibits great smoothness and economy--it is then that you know he's special. 4 years hence, this is the guy who is running consistenly under 4 minutes.
as an aside, i've also seen a 5-11 140lb guy (same size as distance guy) who could run the 110hh under 14 seconds, so the key is matching how their body runs at racing speed with the appropriate races.