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
Cheers
DB
