You can use the Exists static method as follows:
[ADSI]::Exists("WinNT://localhost/Administrators")
To get a True/False result you can wrap into try/catch statement.
$result = try { [ADSI]::Exists("WinNT://localhost/Administrators") } catch { $False }
or in a if/then statement you'll need to wrap it inside a $()
if ( $(try {[ADSI]::Exists("WinNT://localhost/Administrators")} catch {$False}) ) {
write-host "good"
}
else {
write-host "bad"
}