如何区别to是系动词还是介词？？

如何区别to是系动词还是介词？？
我们通常所学到的to都属于介词，后面接动词原形（即to do sth.),；但有时候我们会遇到to 后面接的是动词ING形式或是名词（即 to doing sth;to sth.)，这时，to就是系动词。
例句：There are only two secrets to staying young, being happy and achieving success.
The purpose of his recent trip to Huangzhou was to attend a meeting.

那么平时学习和考试中，我该如何区别to到底是介词还是系动词呢？？？？