There is no guy to do that. That started in Platnum (Gen 4).
If you can get your hands on a Kecleon, go into a Double Battle and use Hidden Power on it. If you don't kill it, it should switch it's type to the of Hidden Power. If it does not work on Kecleon, it's Ghost-type. If it is Super Effective and kills him, it's Fighting-type.
I know Kecleon are not native to FireRed/ LeafGreen, but try to see if a friend can trade you one from Ruby, Sapphire, or Emerald. or hack one.