A quick answer is that the Red Fires have the yellow eyes and chest. The Rubies have red eyes and the chest is pretty much the same as the body. Unless, of course … they are Spectrals or Rising Spectrals. 😀 Those have special eyes that I hate on most pieces but love on these sculpts.
Also, the Red Fires have a lot of yellow/gold mixed in with their red coloring. The Rubies can vary in color a great deal, depending upon the sculpt. Some are very dark red, some wine, some maroon or burgandy. Clear as mud? 😆