To find out whether a given local or domain user/group exists, you can simply use the static method Exists():
[ADSI]::Exists('WinNT://./Tobias1')
This will check whether there is a local account named "Tobias1." To check domain accounts, you can simply replace "." with your domain name, or use LDAP:
[ADSI]::Exists('LDAP://CN=Testuser,CN=Users,DC=YourDomain,DC=Com')
