Test AD User Credentials

Add-Type -AssemblyName System.DirectoryServices.AccountManagement
$DS = New-Object System.DirectoryServices.AccountManagement.PrincipalContext('domain')
$DS.ValidateCredentials($UserName, $Password)
Details
Attempts to authenticate against domain controller and return true or false if it was able to authenticate successfully.

Example
PS C:\> $user = 'user01'
>> $password = 'Password01'
>> Add-Type -AssemblyName System.DirectoryServices.AccountManagement
>> $DS = New-Object System.DirectoryServices.AccountManagement.PrincipalContext('domain')
>> $DS.ValidateCredentials($UserName, $Password)

True
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