If you’d need to find out whether a given Active Directory user exists, and provided you have installed the ActiveDirectory PowerShell module which is part of RSAT (Remote Server Administration Toolkit), here is a clever way:
function Test-UserExists { param ( [Parameter(Mandatory)] [string] $SAMAccountName ) @(Get-ADUser -LDAPFilter "(samaccountname=$SAMAccountName)").Count -ne 0 }
You can adjust the LDAP query to check for users based on other attributes as well.