How do I know if DFS Replication is completed using PowerShell?

You can check the state of DFS-Replication by using the DFSRDiag command.  If replication was in progress, you would see these results
dfsrdiag RepliationState
Updates served:

[1] Update name: user32.amx
[2] Update name: gdi32.amx
[3] Update name: report.docx
[4] Update name: Train.pptx

Total number of Outbound updates being served: 4

Summary

Active inbound connections: 0
Updates Received: 0

Active outbound connections: 1
Updates sent out: 4

Operation Succeeded

This must be ran on a server with DFS installed on it.  Your exact results will depend on what is being transferred.

To do this in PowerShell, I created a function that sends a value of True into the pipeline if DFS replication is active and False if it is not.  Here is the code:

<#
.SYNOPSIS
Test to determine if DFS replication is in progress

.DESCRIPTION
Returns [Boolean]TRUE if replication is in progress.
Returns [Boolean]FALSE if replication is not in progress.

.EXAMPLE
Test-DFSRProgress

True

The example returned True.  That means DFS
replication is in progress.

.NOTES
This function can only be executed on a server
with DFS installed.  It will only test for
DFS replication to and from this executing
server.

.LINK
Invoke-Command
#>

Function Test-DFSRProgress
{
# Execute the MS Dos command "DFSRDiag ReplicatoinState"
    # and store the output in the variable $DFSRInfo.
    $DFSRInfo = Invoke-Command -ScriptBlock {DFSRDiag ReplicationState}

# Test to see if the output has the below strings in it.
    # if it does not, then replication is in progress.
    If ($DFSRInfo -like "*Active inbound connections: 0*" -and $DFSRInfo -like "*Active outbound connections: 0*")
{Write-Output $False}
Else {Write-Output $True}
}