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.
Have been trying to find this out myself for a while, found this post http://communities.vmware.com/message/501161, which helps to some degree. Use the -s option with ls to see the block size so
ls -lh <vmdk path>
should show the size reported by the file and compare this to
ls -1sh <vmdk path>
and of it's smaller than the first value then it's a thin disk