You should have seen his face when he opened the gift.

'should have + 过去分词' 是情态动词 should 的完成式结构。此处用于叙事修辞：说话人以 'you should have seen' 唤起听者对过去视觉的想象，强调表情的有趣程度，而非表达后悔或批评。