Body type

If you want to know whether someone is going to be good at a new sport, take them to a big race and look around. Does their body type match that of the top competitors? If so, chances are they will be fine. If not, better make sure they have some other exceptional attributes to make up for the difference. Or maybe they should go after multi-sport, eg. triathlon.

Runners at the line

Usually, an even easier way to tell is to look at what the person likes as a child. Kids naturally gravitate towards what they are good at. Future athletes tend to stand out at a very young age. They can beat all their friends rather easily and with a huge margin. Occasionally one has to make an exception for speed of development. Some kids mature very quickly and can beat their age-matched peers by virtue of faster development.

These guys are not runners

These rapidly-developing children are the ones that stand out as distance or speed champions well before age 10. Theirs is not so much exceptional ability but rather exceptionally fast development. Their chances of becoming top athletes are not very different from the rest of the population, and once the rest catches up, most of the early birds disappear in the crowd.

Swimmers

So, if you look at kids, don't be tempted by those very young overachievers. Better look at a slightly later age. Phelps's coach claimed he saw the future star as early as age 11 and that makes a lot of sense. Great talent can be spotted early on.

Although there are notorious exceptions, nearly all top athletes in one discipline look alike. They are about the same height, the same weight, and of the same proportions. The latter includes relative length of arms, legs, versus torso, and may include other measures such as chest circumference, head size, shoe size, or hand size.

Body type determines how easy it is to reach things, how much weight you will have to lift, how much air or water resistance you will experience, and how much reach or stride you will have. Head size may influence resistance. Hand and foot size affect how much grip, paddle, or grasp you have.

The most notorious exceptions are people who are too tall or too big for their preferred discipline. Usually these athletes make up for their deficit by exceptional strength, power, or endurance. People like Miguel Indurain, Usain Bolt. They are best known because they don't fit the type. These are the special cases and they are far and few between.