UDPATE: This is a feature built into 6.4 now, so this is no longer really necessary.
Pretty simple Custom Inventory Rule for the K1000 below you can use to tell if FileVault is enabled or not:
It will report whether it's on or off. Now you can make smart labels for which Mac's have it enabled and which one's do not.
If you need instructions for how to create a custom inventory rule: