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(1)从意义的角度来看，同位语从句是名词性从句，与先行词之间存在的是同位或等同的关系，是对先行词的具体内容作进一步的解释说明；而定语从句是形容词性从句，与先行词之间存在的是所属关系，起修饰限定作用。
如：We were greatly pleased at the news that the Chinese women’s volleyball team had won the match again.（听到中国女排再次获胜的消息我们极为高兴。）（that引导同位语从句）
We were greatly pleased at the news that our teacher had told us.（听到老师告诉我们的消息我们感到很高兴。）(that引导定语从句)
(2）从语法角度来看，引导同位语从句的that是连词，只起语法连接作用，在从句中不充当任何成分；而引导定语从句的that是关系代词，它除了起引导从句的语法作用之外，还要在从句中充当句子成分，主要作主语或宾语。
(3）从可否省略引导词来看，同位语从句的引导词that一般不可以省略，但是定语从句中的关系代词如果在从句中充当宾语，常常可以省略。
如：We were greatly pleased at the news (that) our teacher had told us.
(4）同位语从句的先行词多为fact, belief, news, idea, thought, question, reply, possibility, hope, problem, remark, report等名词，引导词常用that，但是并非出现在上述名词后面的从句肯定都是同位语从句，判断同位语从句一定要综合以上原则仔细考虑。
