https://superuser.com/questions/879216/how-to-determine-whether-blu-ray-disc-is-htl-or-lth

lists two methods one with cdrecord + internet, another one using imgburn

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Here's the best way I've found:

  1. Determine the manufacturer code and media type of the media. On Linux, I used cdrecord dev=XXX -atip | grep -i 'manufacturer\|media type', where XXX is the code for the Blu-ray burner as listed by cdrecord -scanbus.

  2. Look up the manufacturer and media type codes at http://www.blu-raydisc.info/licensee-list/discmanuid-licenseelist.php . The table on that site identifies "recording type" (i.e. HTL or LTH) and also write speed from these codes.

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site worked when I clicked on link.

Apparently LTH should be marked due to their bad compatibility with earlier drives, but .... reality is less stellar :(

and in imgburn

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With image burning software. ImgBurn for instance.

In Imgburn main menu just click on anything writing related. For example, "Write image file to disc"

Now a new window pops up

Please check that you have the correct burner selected in the left hand side, if you have more than one disc burner.

Scroll down on the right pane, and you find the info under "Recorded Mark Polarity"

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HTL is apparently more durable....

Terje posted link to Canadian site and I did little search on my own

https://www.pcworld.com/article/423607/hard-core-data-preservation-the-best-media-and-methods-for-archiving-your-data.html
(from 2016)

to be honest I newer heard about HTL vs LTH when it comes to BDs....

I found some ASUS  drive in my city, so *may be* I'll have some means to test real disks in new year.