how to find target is button in Jquery?

You may do only

$(document).click(function(e) {
  if ( $( e.currentTarget ).is( ":button" ) ) {
    // Do things
  }
});

Why would you use :button instead of button?

Because that way you can detect if it's a <input type="button"> OR a <button> tag, aswell the other input types which render as buttons.
If you're unsure about using this selector, check the :button selector jQuery docs.

Faster :

$(this).is('button');