Remove your coolant fill cap, and fill the system up as high as possible. Leave the cap off. Remove your EGI fuse (or disable coils on the carb’d models) to prevent startup. Have an assistant crank the car with the throttle open for several seconds. Observe the open coolant hole for air bubbles or excess pressure pushing water out. Any pressure or air bubbles (consistent) obvserved indicate compression pressure being pushed into the coolant system past the seals, since the water pump is not spinning fast enough to do anything and there is no other explanation. Passing this test does not necessarily guarantee the lack of a seal failure, but it goes a long way in easing my mind. Failing this test pretty much guarantees a failure internally.

Run the car to operating temperature, then shut it down. Refill coolant. Let sit overnight, or at least several hours. When you return, remove your EGI fuse/spark as described above. Get in and crank the car for just a second or 2, enough to amount to just a few revolutions. Stop, and go pull all of your sparkplugs. Any evidence of water/coolant on the plugs means it was pooled up at the bottom of the combustion chamber due to bad seals, and got sloshed up onto the plugs when you cranked it…failing this test guarantees a seal failure, but passing it does not necessarily exclude you from a seal failure.