Script to test AD credentials; heavily based on https://gallery.technet.microsoft.com/scriptcenter/Verify-the-Active-021eedea/view/Discussions#content

Test-ADCredential.ps1

function Test-ADCredential {
  <#
    .SYNOPSIS
    This PowerShell function takes a user name and a password as input and will verify if the combination is correct.
    The function returns a boolean based on the result.

    .PARAMETER Credential
    PS Credential Object.

    .EXAMPLE
    Test-ADCredential -Credential (Get-Credential)
    Returns a True if these credentials work against AD

    .LINK
    https://gallery.technet.microsoft.com/scriptcenter/Verify-the-Active-021eedea/view/Discussions#content

    .INPUTS
    PS Credentials Object

    .OUTPUTS
    Boolean
  #>

  Param
  (
      [Parameter(Mandatory,HelpMessage='AD Credentials', ValueFromPipeline)]
      [System.Management.Automation.Credential()]
      [pscredential]$Credential
  )
  begin {
      Add-Type -AssemblyName System.DirectoryServices.AccountManagement
  }
  process {
    if (!($Credential.UserName) -or !($Credential.GetNetworkCredential().Password)) {
      Write-Warning -Message 'Test-ADCredential: Please specify both user name and password'
    } else {
      if ($Credential.UserName -match '@') {$user = $Credential.UserName.Split("@")[0]}
      elseif ($Credential.UserName -match '\\') {$user = $Credential.UserName.Split("\\")[1]}
      else {$user = $Credential.UserName}     
      $DS = [DirectoryServices.AccountManagement.PrincipalContext]::new([DirectoryServices.AccountManagement.ContextType]::Domain)
      $DS.ValidateCredentials($user, $Credential.GetNetworkCredential().Password) #NB: Username should not contain domain prefix (i.e. use myUsername instead of myDomain\myUsername)
    }
  }
}