By using commandname property we can find the button is clicked,
When we have multiple Button controls on a Web page, use the CommandName property to specify or determine the command name associated with each Button control. You can set the CommandName property with any string that identifies the command to perform. You can then programmatically determine the command name of the Button control and perform the appropriate actions.
Additional information about the command, such as Ascending, may be specified by using the CommandArgument property.