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.