The 'Superman reveal' is one of the few things about the theatrical cut of Superman II
(along with the "General, would you care to step outside?" line) that's preferable to the Donner cut.
(In the theatrical version, Clark stumbles and accidentally moves his hand into a fireplace, and that's what makes Lois realize he's Superman. In the Donner cut, she purposely tricks him with a gun loaded with blanks.