How did the watchman at Jezreel recognize that the approaching charioteer was Jehu?

Answer: He drove like a madman

The watchman said the charioteer was driving like Jehu son of Nimshi, "driving like a madman" (KJV: he driveth furiously).

“Again the watchman reported, “He reached them, but he is not coming back. And the charioteer is driving like Jehu son of Nimshi—he is driving like a madman!””— 2 Kings 9:20 (BSB)