You can use the Optimization and Migration capability to scan VMFS datastores and see which ones are optimized.
About this task
As part of the scan process, the Optimization and Migration capability takes VMware snapshots of the virtual machines that are powered on. As soon as the snapshots are no longer needed, it quickly deletes them to free up that space.
This is briefly described on page 105 of the Virtual Storage Console for VMware vSphere Installation and Administration Guide.
Martin is correct that VSC invokes a vSphere snapshot which is required for powered-on virtual machines on VMFS datastores. This is also why powered-on virtual machines with independent persistent disks on VMFS can't be scanned. (vSphere snapshots can't be created.) vSphere snapshots are also created for VMs on NFS datastores if backed by non-NetApp storage or if the NetApp storage system has not yet been added or discovered under Monitoring and Host Configuration.