Checking domain name availability with DNS records
The only way to determine if a domain is available or not is to query the registry database for that domain. Usually you can do that by performing a WHOIS query on the registry WHOIS interface.
There are several reasons to explain why a DNS check is not safe.
The most simple explanation is because you can register a domain without pointing it to any location. It's like a house. You can buy a house, but you are not forced to build a road to reach it.
The most part of available domain providers force you to point a DNS, otherwise they set a default one. But you're not forced, it's not a kind of mandatory domain requirement.
The second reason is because there are actually cases when a domain is registered but not available. One of this cases is the period often known as redemption period. This period lasts for some days after the domain is expired and the current owner don't renew it. Several registries disable the domain, it means it is no longer reachable via DNS, but the domain still exists and can be redeemed (usually) only by the original owner.
The DNS check is cheap and sometimes faster. If you need to check whether the domain exists, then you can try a DNS query first and, in case of failure, fallback to the WHOIS query. In other words, if the DNS query succeed the WHOIS query is almost pointless. But you cannot do the opposite because if a DNS query fails, it doesn't mean the domain is not registered.
The only 100% reliable way to check for domain availability is to query the registrar's database. I wouldn't trust whois.
You can use DNS to get an estimate. Instead of looking for an SOA record, I would just look to see if anything at all is listed with the TLD name server.
dig is a good tool for this (runs on Windows, too), although I guess you could use nslookup too. For example:
dig co.za. NS
will provide a list of the name servers for .co.za. One of those servers is ns0.is.co.za.
Next, query that server directly to see if they have anything listed for your domain of interest:
dig ibm.co.za @ns0.is.co.za
That query returns NS records, but no SOA record since SOA records are provided by the domain's name server (which may or may not be online). The NS records indicate the domain name is in use.
The reason for going direct is that it's usually much faster than relying on recursive queries from your local name server.