Do a compression check on the smoking bank. Check against the entire engine. Do it when the engine is cold and make sure ALL of the plugs are out. 7 or 9 turns, but use the same. I like 7.
How many miles or hours are on the engine? If it is a fresh rebuild and it is not broken in, you can expect a certain amount of
oil burning.
If you get a good set of reading on the compression, leave the plugs out and pressure check the coolent system to 17 - 20 PSI. At cold it could have a very small coolent leak into the combustion chamber that seals when it gets hot ( seen this before ).
If it is good to go on these two tests, pull the rocker covers and pop a new set of seals on. Should take about two hours, maybe four. While the spring(s) are off, you can drop the valve just a little and check for wobble. If you have large wobble in the valve stem, it will puff like a steam engine.
If all else fails, do a leak-down test on each cylinder.
If it is burning blue, it is
oil and that means it is getting into the combustion chamber. If it is white, it is coolent and there is a leak into the combustion chamber.
Hope this helps.