Unfortunately, there is no good way to test. If rings were the problem leak down testing will not test the oil rings. It will test the compression rings though and if you have a uniform carbon condition on all plugs (not caused by V/G seals), it will be because oil is being pushed into the intake through an overwhelmed PCV system. Definitely check your PCV system. Definitely check that the flow is in the proper direction. There is usually a hose from the airfilter (on most cars) to the valve cover. Air flow in this hose is from the filter to the engine (NOT THE OTHER WAY). If it is the other direction then you have either too much blow by or not enough scavanging (PCV).
I don't know enough about Saabs to state statistics but on MBs the answer to your question would be to replace the V/G seals as the test. On most engines it is easier than any realiable form of testing and the seals are cheap.
Bosch Master, ASE Master, L1
33 years MB technician