If you want to see who has potential as a sprinter, jumper or shot putter then have them sprint, jump and put!
A simple test day of: 50 meter dash, Long jump (2 attempts), Shot put (2 attempts) will go a long way in telling you what you need to know. Show them the basic basic movement then let them go for it. Don’t expect anything to look pretty, especially the jumps and put, but check their marks and watch for the ease in which they do it.
As you accumulate data and spend more time around youth you’ll start to recognize potential in just watching kids play and move and how they are built. The common factor seems to be the ability to deliver power, there isn’t always much at this age but the ones who have potential have some explosiveness to them, they pop off the board or the shot put springs out of their hand.