You need a "leak-down tester". In fact you could check both engines with it. Here are two home made units I just found on the internet. [img]http://www.lafishmag.com/LeakTester.jpg[/img] Home made unit in photo
Another home made unit
Here is one that uses two gauges and has instructions to use even if you don't have a compressor that can maintain 100psi.
Longacre racing $100

'67 GMC 3/4 292 4spd