check group policy templates - compare-ADMX.ps1

How to find missing admx or adml files - of course - You should provide proper path and regional settings:

import-module ActiveDirectory

cls
$currentDomain = [System.DirectoryServices.ActiveDirectory.Domain]::GetCurrentDomain()
$currentDomain = $currentDomain.Name

$admls = get-item ("c:\windows\sysvol_dfsr\sysvol\" + $currentDomain + "\Policies\PolicyDefinitions\en-us\*.adml")
$admxs = get-item ("c:\windows\sysvol_dfsr\sysvol\" + $currentDomain + "\Policies\PolicyDefinitions\*.admx")

$admls | %{
    $adml = $_
    $admlName = $adml.name
    $admlFullName = $adml.FullName
    $admxName = $admlName.Substring(0, $admlName.Length - 5) + ".admx"
    if (Get-Item ($admlFullName + "\..\..\" + $admxName) -ErrorAction SilentlyContinue) {
        #do nothing 
    } else {
        ".......... " + $admxName + " is missing"
    }
}

$admxs | %{
    $admx = $_
    $admxName = $admx.name
    $admxFullName = $admx.FullName
    $admlName = $admxName.Substring(0, $admxName.Length - 5) + ".adml"
    if (Get-Item ($admxFullName + "\..\en-us\" + $admlName) -ErrorAction SilentlyContinue) {
        #do nothing 
    } else {
        ".......... " + $admlName + " is missing"
    }
}