認得當年 He Knew Him Of Old
那國丈聽見，急睜睛仔細觀看，見那和尚變了面皮，不是那般模樣。咦！那国丈听见，急睁睛仔细观看，见那和尚变了面皮，不是那般模样。咦！
逐句解 Line by line
When the Royal Father-in-Law heard this he opened his eyes wide and looked closely, and saw that the monk had changed his face and was not of that former appearance at all.
仔細 is zǐ xì — 仔 is zǐ here, a standing ruling for this edition, not the zǎi that a modern dictionary gives first. 模樣 is mú yàng: the first character is mú when it means the look or shape of a thing, not the mó of 模型, a model. It is the same word behind the everyday 像模像樣, "looking the part."
Aiya!
咦 is the storyteller clearing his throat. It is a one-character interjection that arrives just before a revelation, and its job is to make the audience look up. There is no good English equivalent; the sound itself is the meaning.
He knew him for the Great Sage Sun of years ago, / whose old fame was won five hundred years before.
The couplet is doing the work a film would give to a cut and a music cue. Five hundred years is the exact span the novel has been counting since Wukong was pinned under the Five Phases Mountain, so the line is not rounding: it is the book's own clock. 認得 takes dé, the reading this edition uses after a verb of perception.
This is the hinge of the chapter and it takes three lines. A look, a shout, a couplet. From here the Royal Father-in-Law is not a court favourite arguing medicine; he is a demon who has been recognised, and everything that follows is a chase.
The verse is the novel's standard device for a recognition, and it works because it changes who is speaking. Prose is the narrator; a couplet like this is closer to a chorus, stepping forward to tell the audience what they have just seen. Whatever the Royal Father-in-Law thought he was fighting, the poem has now named it.