Try the following method, I think that is more like the Web Client is showing.
$vmName = 'MyVM'
$tgtDSName = 'MyDS'
$vm = Get-VM -Name $vmName
$ds = Get-Datastore -Name $tgtDSName
$si = Get-View ServiceInstance
$vmProvCheck = Get-View -Id $si.Content.VmProvisioningChecker
$spec = New-Object VMware.Vim.VirtualMachineRelocateSpec
$spec.Datastore = $ds.ExtensionData.MoRef
$result = $vmProvCheck.CheckRelocate($vm.ExtensionData.MoRef,$spec,$null)
$result | %{
$_.Error | Select @{N='Message';E={$_.LocalizedMessage}}
}