口說無憑 Talk Proves Nothing You Go First
妖王道：「鈴兒乃金丹之寶，又不是飛禽走獸，如何辨得雌雄？但只是搖出寶來，就是好的。」行者道：「口說無憑，做出便見。且讓你先搖。」妖王道：「铃儿乃金丹之宝，又不是飞禽走兽，如何辨得雌雄？但只是摇出宝来，就是好的。」行者道：「口说无凭，做出便见。且让你先摇。」
逐句解 Line by line
The fiend king said, "Bells are treasures of the golden elixir, not birds of the air or beasts of the field. How is one to tell cock from hen?
得 is de here, the potential — how could one manage to tell. The pair 如何得 would take the other reading, but 辨 stands between them, so this one has to be judged on its own.
"Whichever shakes out its treasures is the good one, and that is all."
He proposes a test, and it is a perfectly fair one. 但只是 stacks three limiting words — only, just, is — for the sense of never mind all that, the point is.
Pilgrim said, "Talk proves nothing; do it and we shall see. You go first."
口說無憑 is a set phrase — spoken words are no warrant. The five characters that close the quotation are short enough to ride in the same segment as the sentence that opens it.
The fiend wins the argument and loses everything. He correctly points out that bells are not livestock and cannot be sexed, and proposes the only sensible test there is: shake them and see which works. He has reasoned his way, step by careful step, to the exact action that will finish him.
口說無憑，做出便見 — talk proves nothing, do it and we shall see — is Wukong agreeing enthusiastically. He has spent this whole passage being invited to produce evidence, and the evidence is on his side, because his bells are the real ones.
And then 且讓你先搖: you go first. The politeness is the point. He is letting the monster demonstrate a set of counterfeits in front of a crowd, and he offers it as a courtesy.