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