Can you make this statement more precise and give references to support it? I'm not sure what you mean by "indicators."

I mean that if you were trying to select 100 kids to ensure that at least one would go on to win a Fields medal, picking out those at the top of the Math Olympiad distribution (the highest gold medal scores, youngest gold medalists) would probably suffice.

I've heard this from many people and believe it but have not had a chance to chase down references - do you have some handy?

The biggest datasets are from militaries (with the most representative ones from countries with universal conscription). Check out the APA report on intelligence by Niesser et al, this zoo of references on wikipedia, or Linda Gottfredson's website.