Compression test.
1. Record the compression in each cylinder for a base value and CHECK with the manufactures recommended compression results.
2. If low or on the lower limit of the manufactures compression figures, add a small amount of oil into each cylinder and check compression
3. If there is a large increase in compression, then rings are gone.
4. If a small/minor increase, stem seals are worn.
But without that, generally if your engine blows smoke only after idling for a bit it will be valve stem seals (and sometimes a bit of top end to). Generally if under load, usually rings.