So we do track guest file system used / free via vmtools and those metrics are available. Easiest way I can think of is to build a custom heatmap in the vSphere UI showing VM allocation by guest FS free space. The larger and "red'er" a VM's box the better a candidate for thin provisioning - I think.... all the stuff in my home lab is thin provisioned so I'm trying to test this out - you will notice that even with thin-provisioned you could have a big delta between guest free and VM used.
Here's how I set it up....
And here's my heatmap, before I have thickened any VMs - I'll follow up after I thicken a few up. Basically the BIGGER the VM box the more datastore it is consuming. The red means high amount of free guest OS file system space.
I think that a custom report could also address this. I'll try to work on that.