Use an air compressor with an adapter. Bring the piston up so the exhaust port is closed, try to pressurize it around 100 psi and see how fast it leaks down. Might give you an idea. If everything else is that bad I'd guess there's not much in compression.
If you pull off the exhaust you can look at the exhaust side of the piston, that's generally where you'll see damage to the ring or piston