While not 100%, you can look at the following to see what control caused the postback on a form:

Request.Form("__EVENTTARGET")

It works because ASP.Net uses javascript to handle the postback functionality, and the __EVENTTARGET query string value can contain the name of the control which caused the postback.

Want to know what arguments are being passed during the postback? Take a look at the Request.Form(”__EVENTARGUMENT”) value!