You can scan all VMs, get their VMDK and check if the same one appears more than once.
A quick way to do that could be
Get-VM | Get-HardDisk |Group-Object -Property FileName |
where{$_.Count -gt 1} |
Select @{N='VMDK';E={$_.Name}},@{N='VM';E={$_.Group.Parent.Name -join '|'}}