I just tried it

.

The only indication is

ddb.thinProvisioned = "1"

in the descriptor if it is a thin disk.

Eagerzeroed, zeroed and thick disks have the same descriptor.

Eagerzeroed and zeroed disks (of the same size) even pass a binary comparison.

Thick disks of course don't pass the binary comparison due to their nature.

The only logical answer to me is that part of the CID value distinguishes them.