After the Kentucky Derby has been run, it is usually easy to assess a winner's talent. There was no doubt, for example, that Spectacular Bid was a champion, that Gato del Sol was merely the best of a very bad lot.

But it is still rather difficult to judge the hero of the 109th Run for the Roses. Sunny's Halo won on Saturday with complete authority--much more than his two-length margin would suggest--and remained unbeaten as a 3-year-old. He scored his victory, however, with the aid of perfect racing luck, just as he had done in the Arkansas Derby three weeks earlier.

Everyone had expected that Sunny's Halo would have to cope with a hot early pace, which almost always develops when 20 horses go charging to the first turn at Churchill Downs.

Instead, he found himself sitting in second place behind a pacesetter who ran the first quarter mile in a very slow 23 4/5 seconds; he got perfect position without having to exert himself.

That the slow pace helped the leaders was proved irrefutably by the performance of Desert Wine. The California colt was clearly unsuited to the Derby distance--he didn't have the breeding and he had never won beyond 1 1/16 miles--and yet he managed to finish second. A detractor could point out that there is nothing too overpowering about beating Desert Wine by two lengths at 1 1/4 miles. And the time of the race, 2:02 1/5, was not exceptional, either.

But if Sunny's Halo was the beneficiary of good fortune on Saturday, he is the type of horse who may keep on getting lucky--because he has the ability to make his own breaks.

He has the the most valuable of equine traits: controllable speed that a jockey can use when he wants. Affirmed had it; he wasn't a better racehorse than Alydar or Spectacular Bid, but he beat them because his speed and versatility always gave him a tactical advantage over his arch rivals. Sunny's Halo may not be so superior to Desert Wine, Caveat and Marfa, but he may keep beating them nevertheless.

Sunny's Halo wasn't born with this tractability. Trainer David Cross has worked hard to develop it.

"We try not to do anything to put him on edge," Cross said this morning as he savored the great achievement of his career. "We don't give him any short three-furlong workouts. We don't change his routine on the day of the race. And he's become very good mentally. When they played 'My Old Kentucky Home' it didn't bother him a bit."

With his speed, Sunny's Halo will be a tough horse to beat in the Preakness, even though the trainers of Marfa and Caveat know that a different type of pace could favor their stretch-running horses. In fact, there was nobody on the backstretch today who seemed more optimistic about the future than Caveat's trainer Woody Stephens: "He definitely would have been a winner in another sixteenth of a mile," the veteran said.

Cross has mapped out the rest of his colt's campaign as meticulously as he planned the steps that led to the Derby. He insists that he will bypass the Belmont Stakes: "I don't like 1 1/2 miles in June with 126 pounds and I don't like New York." Instead he will run Sunny's Halo in the Queen's Plate at Woodbine, which is important to his Canadian owners.

Then he will go to the Swaps Stakes at Hollywood Park, the Travers at Saratoga--"if they let me back in New York"--and the rich Super Derby at Louisiana Downs.

In that succession of tests, Sunny's Halo surely will encounter a variety of challenges that will demonstrate how he copes with adversity. Then we will know what we couldn't fully deduce on Saturday: whether he is a very good, versatile racehorse or whether he was very lucky at Churchill Downs.