How do Siddhartha and Govinda recognize Gotama?

From the text:

Siddhartha saw him, and he instantly recognised him, as if a god had pointed him out to him. He saw him, a simple man in a yellow robe, bearing the alms-dish in his hand, walking silently.

"Look here!" Siddhartha said quietly to Govinda. "This one is the Buddha."

Attentively, Govinda looked at the monk in the yellow robe, who seemed to be in no way different from the hundreds of other monks. And soon, Govinda also realized: This is the one. And they followed him and observed him.

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".... the Buddha walked, wore the robe and placed his feet just as all of his monks did, according to a precise rule. But his face and his walk, his quietly lowered glance, his quietly dangling hand and even every finger of his quietly dangling hand expressed peace, expressed perfection, did not search, did not imitate, breathed softly in an unwhithering calm, in an unwhithering light, an untouchable peace."