Checking domain name availability with DNS records

Question

How accurately can a domain name's availibility be determined by checking for the existence of NS or SOA (Start of Authority) records?

If not, how can I determine this as accurately as possible without resorting to WHOIS? For example, is it worth checking for MX records if no NS records exist?

I am specifically interested in .co.za domains, but the only official, fool-proof way to check availability in the .co.za namespace is to use http://co.za/whois.shtml, which limits consecutive checking by IP address.

Answer 1

• Whois is the only 100% sure way of checking for .co.za domain availability. See http://co.za/whois.shtml
• My tests show that checking for the existence of NS records or ANY DNS records in bulk seems to be the most accurate way of guessing whether a .co.za domain is available for registration. SOA records work too, but not as well.
• dig co.za. NS shows .co.za TLD nameservers. Ping TLD servers and use the closest ones to avoid unncessary recursive queries.

Thanks for your answers.