Lisle Tools makes a combustion gas leak detector for cooling systems to check for blown head gasket. The rubber cone on the device would need to be adapted to the A rad. neck. You would want to temporarily plug the outside end of the overflow tube. Once inserted into the rad. neck, and the special test fluid added, you apply a vacuum for two minutes. Then check for a color change from Blue to yellow= blown gasket. Test over.
It is very hard to see bubbles in an A rad. with the baffle in the way unless the leak is huge.
http://www.lislecorp.com/divisions/p...n=1&category=8
I diagnosed 6 blown head gaskets this summer: 2 on As and 4 on modern cars