"The Man Who Always Knew", a short story by Algis Budrys; first published in Astounding Science Fiction, April 1956, available at the Internet Archive.
"Gosh, Mr. McMahon. I don't know what to say. You mean you travel around the country just looking for people that are working on something new?"
The small man shook his head. "No. I travel around the country, and I stumble across people who're going to accidentally stumble across something good. I've got secondhand luck." The small man rolled the bill up between his fingers, and smiled with a hurt twist in his sensitive mouth. "It's even better than that. I know more or less what they're going to stumble across, and when they're going to." He bent the tube he made out of the bill. "But I can't develop it myself. I've only got one talent."
"Well, gee, Mr. McMahon, that's a fine thing to have."
The small man crushed the dollar bill. "Is it, Harry? How do you use it directly? How do you define it? Do you set up shop as McMahon and Company—Secondhand Luck Bought and Sold? Do you get a Nobel Prize for Outstanding Achievement in Luck?"
"You've got a Nobel Prize, Mr. McMahon."
"For a cold cure discovered by a pharmacist who mislabeled a couple of prescriptions."
"Well, look, Mr. McMahon—that's better than no Nobel Prize at all."
[. . . .]
"Two Whiskey Sour, and another Gibson," the waiter said. Harry moved unhappily down the bar and began to mix, thinking about Mr. McMahon. Then he heard Mr. McMahon get off his stool and come down the bar.
He looked up. The small man was standing opposite him, and looking down at the bar. Harry looked down too, and realized he'd been trying to make a Whiskey Sour with Gibson liquor. It looked like nothing he'd ever seen before.
Mr. McMahon pushed the dollar bill across the bar. He reached out and took the funny-looking drink. There was a sad-happy smile on his face.
"That's the one I wanted, Harry," he said.