How do you determine what element of a form is the one responsible for an autopostback in ASP.NET VB?

I have a 开发者_JAVA百科number of dropdown lists that I'm trying to chain together and they all have autopostback. How can I tell which one of the dropdown lists was the one responsibe for the autopostback? I'm very new to ASP.NET and visual basic so any suggestion, no matter how elementary, is appreciated.

There is an entire blog post about how to do exactly this.