While it's not foolproof, you'll find that placeholder VMs don't return anything when you do a Get-HardDisk on them so you could try something like this.
Get-VM | ? {$_.PowerState -eq "PoweredOff" -and ($_ | Get-HardDisk) -eq $null}
I believe this will return all your placeholder VMs although it's not foolproof because if you happen to have a regular VM that doesn't happen to have a hard disk attached it too will show up. I haven't been able to find any other way to differentiate a placeholder VM yet.