A reliable, low cost device for determining when dangerous levels of hydrogen gas have been generated in a transformer is disclosed. The hydrogen indicator is defined by a module assembly that threads into either the headspace or into the oil-filled body of a transformer. The module has an open interior that contains a film that incorporates a hydrogen-sensitive chemochromic indicator. The indicator film is visible through a lens. When the film has been exposed to hydrogen, chemical changes in the chemochromic indicator cause the film to change color—the color change is immediately visible through the lens.