It isn't any of the usually popular signatures-I.e. brock/misty/ash/meowth/team rocket/ken sugimori I can tell you that much.
It just be a red squiggle, and to be honest even if we did find out whose signature it is it'd still be worth nothing without a certificate of authorization.
The Charizard is a very nice find though, it'll sell for more currently now as there is a lot of Charizard hype