只要分析句子成分，就可以区分了。

一、as引导原因状语从句，在句中充当状语，是连词， 如：

As her mother was ill, she was late for class. 因为。。。

二、as 引导非限定定语从句，在从句中充当主语或宾语，是关系代词，如：

As is known to all, Taiwan is part of China.

As 在从句中充当is的主语，关系代词，指代主句所陈述内容。 又如：

As we all know, Taiwan is part of China.

As 在从句中充当know的宾语语，关系代词，指代主句所陈述内容。