So you opened the radiator and there was no oil mixed with coolant? White means there's definately coolant in your system. If there's no mixed coolant with oil (it'll look milky), then might not be a head gasket. Do a compression test or even a leak down test would be better. Does your car over heat or run hotter than it usually does? Do you smell fumes and does it smell sweet?

Are there any mods to the car? Does the white smoke have a blueish tint to it? If it only happens during hard acceleration you might be running lean on the top end.