Use the Page.Request.Params(”__EVENTTARGET”) to get the name of control who caused postback.
However, this does not return the name of control,if the postback is caused by button/ image button.
You can get the button control by looping through all the controls from Page.Request.Forms and comparing with Button Webcontrol.

Dim controlName as string
controlName = Page.Request.Params(”__EVENTTARGET”)
Dim control as Control
If controlName <> Nothing And controlName <> String.Empty
control =Page.FindControl(controlName)
End If

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