//logic to identify your update panel Id if there are multiple update panels on your page.

 If scriptManager.AsyncPostBackSourceElementID.Equals(upUser.UniqueID) Or 

 //logic to identify your the control that generated this post back.

scriptManager.AsyncPostBackSourceElementID.Equals(cmdFilterUser.UniqueID) Or 

//if you want to identify a specific element exist in this panel like in my case there is gridview naming "ngvAuditHistory"

scriptManager.AsyncPostBackSourceElementID.Contains("ngvAuditHistory") 

Then
                        If Request.Params("__EVENTARGUMENT") = "0" Then
               //your logic for the __dopostback response

a way to identify your specific update panel on the page on server side

VB.NET