If the issue is valve stem seals, you will find that the smoke is greatest when opening the throttle after idling for a few minutes, but it will decrease when running under load, where if it is rings, expect the smoke to increase with full throttle under load. Of course, it may be both. And check spark plugs for signs of oiling - if one plug is oily, it is likely to be rings on that cylinder. All plugs oily can indicate general wear - or the wrong heat range plugs!
A compression test is likely to give a good indication of ring wear, although there is always the possibility of the compression rings being ok but a broken oil control ring. There is also the possibility of the cylinder walls being glazed.
Unfortunately, while compression testing and borescope etc will give an indication as to the problem, the only definitive answer will be to pull the pistons and look at the rings.
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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