From my experience you will see the shakes in the log before it is sawn it will show in the butt end or top, you can see the growth rings are not stuck together. So if you don't see shake present in the ends or slabs lifting on the faces in the timber you will most likely have fine timber. I suspect is has something to do with soil conditions the trees grow in. I have sawn 2x stock from hemlock and if it is dropped from waste high onto the floor they will break into two or three sections.
