You can also see from the vSphere web client. Just check the VM hardware settings as seen here:
In my example the disk is Thick provisioned do take a look as see what yours says. I'm not aware of anything that would change the format of a VMDK just by attaching and detaching unless something was run inbetween.
As a.p. and continuum have mentioned - it is totally possible that your Thin provisioned disk has expanded to the full allocation.
Anywho - check the above and then revert back
Kind regards.