最简单的方法：看从句缺什么成分.
比如：
1.He is the policeman who caught the thief the other day.
去掉who后,从句缺主语：caught the thief the other day.所以who充当主语.
2.He is the policeman that I met yesterday.
去掉that后,从句缺宾语：I met yesterday.所以that充当宾语.
3.I like the book whose cover is dark green.
去掉whose后,从句缺定语：(Its) cover is dark green.故whose充当定语.
4.The temple where he used to live has been destroyed.
去掉where后,从句缺状语：He used to live (in the temple).故where充当状语.
如果仅仅是想区分主语或宾语,还有一种简便的方法,就是看关系词后是什么词性的词,后面是名词（主语）的话先行词就是宾语,是动词（谓语）的话先行词就是主语.
如：
1.Father is talking to the man that has stayed here for a long time.
句中that后是谓语动词,所以that就充当主语,可以用who来代替.
2.Father is talking to the man that we are afraid of.
句中that后是名词（主语）,所以that就充当宾语,可以用whom来代替.