你可以去章振邦《新编英语语法》P995，里面有关于介词词组句法功能的歧义，当介词短语位于句尾，里面有三种方法来简单的辨别，1.“位移法”（介词词组移到句首测试是否是状语）2.“被动法”（宾语+介词词组放在句首构成被动测试它是否做名词修饰语）3.“to be嵌入法”（宾语和介词词组之间插入to be测试是否是宾补）；有时候to be嵌入不合适，可以将宾语放句首构成被动语态，此时介词短语作主语补语，逻辑上和宾语与宾补之间的关系一致。
I found him at work.
At work，I found him.(=when I was engaged in working, I found him,我在工作时发现了他)
He at work was found by me.(=He who was engaged in working was found by me.我发现他正在工作)
He was found at work by me.和上面意思差不多。
如果句义都可以说得通，就说明这个句子有歧义。
