There's actually no good way to query for this information, even using the vSphere API, unfortunately to really figure out whether a VMDK was created using "eagerzeroedthick" or "zeroedthick" you need to query the vmware.log files located within the VM in question.
Take a look at this blog post by Mike Laverick - http://www.rtfm-ed.co.uk/2009/12/02/what-i-learned-in-the-last-couple-of-weeks/ which explains how to get this information, it's 2/3 down in the post.
Here is a script that I wrote that queries for this information in an automated fashion -
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William Lam
VMware vExpert 2009
VMware ESX/ESXi scripts and resources at:
VMware Code Central - Scripts/Sample code for Developers and Administrators
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