2001 Chevrolet Truck engine block test

My serpentine belt came off due to the belt tensioned locking up and I had to drive the truck about 10 minutes before shutting down the engine, had to be towed to a local shop, they put a new tension pulley and belt on and did what they called a "block" test and said that it showed that exhaust was mixing with coolent, now I'm looking at a $2,600.00 repair bill to have the heads sent out to a machine shop to see if they are warped or cracked or if it was just a blown head gasket and have the whole thing put back together after they determine what is the specific problem with the heads. Is this engine block test an accurate enough tool to be able to determine that much about whats really going on or not?