This script will allow you to check the Active Directory backup date on all the domains of your forest.
Each domain is checked by selecting a domain controller that is running the service ADWS (Active Directory Web Services).

You can adjust the variable $backup_age_threshold if you want a different backup age threshold.

$myForest = [System.DirectoryServices.ActiveDirectory.Forest]::GetCurrentForest()
$forestDN = (($myforest.Name).split(".")|%{"DC=$_"}) -join ","

$domain_list = $myForest.Domains.Name
$domainControllers = $myForest.GlobalCatalogs.Name

$array = @()
$backup_age_threshold = 2

$domain_list | % {
    $domain_fqdn = $_
    $dcname = Get-ADDomainController -Discover -Service ADWS -DomainName $domain_fqdn | select Hostname -first 1 | % { $_.Hostname }
    $Partitions = (Get-ADRootDSE -Server $domain_fqdn).namingContexts
    $Partitions | % {
        $Partition = $_
        $object = Get-ADObject -Identity $Partition -Properties msDS-ReplAttributeMetaData -Server $dcname
        $Object."msDS-ReplAttributeMetaData" | ForEach-Object {
            $MetaData = [XML]$_.Replace("`0","")
            $MetaData.DS_REPL_ATTR_META_DATA | ForEach-Object {
                If ($_.pszAttributeName -eq "dSASignature") {
                    $backup_date = Get-Date $_.ftimeLastOriginatingChange -format "yyyy.MM.dd HH:mm:ss"
                    $backup_age = ((Get-date) - (Get-Date $_.ftimeLastOriginatingChange)).TotalDays
                    $Properties = @{domain=$domain_fqdn;dc=$dcname;partition=$Partition;backup_date=$backup_date;backup_age=$backup_age}
                    $Newobject = New-Object PSObject -Property $Properties
                    $array += $Newobject
                }
            }
        }
    }
}
$array | % {
	if ($_.backup_age -gt $backup_age_threshold) {
		write-host $_.domain ($_.dc) / Partition $_.partition / Backup is older than the configured threshold ($backup_age_threshold days) (last backup occured on $_.backup_date) -foregroundcolor red
	}
	else {
		write-host $_.domain ($_.dc) / Partition $_.partition / Backup is OK (last backup occured on $_.backup_date) -foregroundcolor green
	}
}