So to answer the question, I don't think there is a definitive way to verify the transmissions health other than to have someone familiar with HST in the brand you are considering drive it and say "it seems sloopy" or "it seems normal as mine". I often want to find folks with an "old" HST and drive it to see how it performs after say 2000 hours.
I agree, it is tough to verify internal health of the HST, other than it works
or it does not.
Years ago when I was hunting for my 1st tractor, I was worried about the
health of a used HST, too. I knew nothing about their reliability or how they
failed. I assumed costs to replace/rebuild were much higher than for
anticipated failures in a gear tractor. My assumptions were mostly wrong,
as I have learned since then, after owning, operating, and fixing numerous
HST tractors. The reality is a used and abused tractor with a gear trans
is likely to need a new clutch, which a very labor-intensive job. HSTs have
proven to be VERY reliable, as long as they have plenty of clean hyd fluid.
HSTs are quite fixable, too, and may not cost as much as you think.
My B21 project had 2400hrs or so on its HST, and it was is great shape. I
am about to crack open another HST on a project tractor...this time a JD
4300, the model that came just before the "eHydro".
RAT, tell me more about your comparison of the L48's HST with the Grand L's
HST.