He must mean by looking at them.
A bloke calling himself Prophet on the forum at RacingandSports, reckons he can tell how fit a horse is by looking at it.

I go to the races every 2nd week, yet when I look at them, most look fit - after all they are all racehorses.

One thing that you're supposed to look for is six pack chest. Shogun Lodge is a good example - when he's fit he looks very muscular, when he's not he looks heavy.

You need a trained eye I guess to be really good at it.