It's multifaceted and different between sports. So for example, in track and field I can measure talent by the absolute performance attainable by an athlete, like a 10.0s 100m guy, or a 9.8s guy or a 10.2s guy. In something like judo or jiujitsu, or some other martial art we might talk about 'world champion' level, or 'world class' or 'national class. It's harder in sports with more skill.
Now if we are talking specifics, a great sprinter will have small feet, narrow hips, high calves, large muscle bellies, lots of fast twitch fibers, and great tendon attachments. But the absolute talent of an athlete can also be affected by things we can't measure by looking at them, like rate of improvement. You find kids all over the country that are fast, but don't improve year to year. Some of that is bad coaching, and some of that is 'lack of talent.' More specifically, they are already close to their maximum potential.
If you're around elite runners, after a while you notice they all look the same. When it comes to grapplers, you have a wider variation in what is acceptable, since you can use different techniques to fit your body type. Though you typically find grapplers are short (for their weight), strong (especially in the upper body), with wider hips, and t-rex limbs. Large muscle bellies are a must for almost any 'power' sport. Other things also start to matter like coordination, initiative, and spatial awareness. These things are harder to measure by looking at them, but you might see them with a hidden talent for tetris.
