Back in the 70's I designed a plate to attach to Vega cylinder head, that you could pressure up with an air hose 1/4 inch nipple. It had a shut off valve with a pressure gauge. Then you put it in a pan of hot water to see any bubbles or lost pressure you know the head was cracked. If you use the same idea of lost pressure, you could talcum powder and repressure then tear down to see if the leak was gasket or valve seat. You know if you have lost pressure your going to tear down. You could even let it set over night for any pressure lost. Can't think of any other way to find the leak. I used foot powder spray on trans or oil pan gaskets to find leaks. Clean area spray powder on watch for leak.
Brad