ah, same happened to mine except i had spewed all the coolant out the cap versus a crack in the rad. I'm pretty sure that's when i finished blowing the head gasket, once i let it run without any coolant i had no idea it had thrown it all up when i parked it at the store and came back to it hours later.
Anyways, the most concrete proof would be in a compression test, that'll tell you if you have loss of compression which could be due to a blown gasket (among other things...).