Define "non-existent" domain, then.
From the context of a DNS request it is impossible to differentiate "existent" from "resolvable". A domain that does not resolve (no dns server, etc.) may still "exist" from a registrar's point of view. A domain that does not have an A record is just a domain that is misconfigured, not one that "does not exist"; and still, in the context of a DNS they are both pretty much the same.
Realize that the purpose of mIRC's DNS functionality (and the functionality of DNS in general) is to convert from a host to an IP or an IP to a host, nothing else (besides MX, which mIRC clearly has no need to support). In this sense, it's irrelevant whether or not a lookup is NXDOMAIN or has no A record, since both responses will fail to resolve in the context that matters (host->ip conversion).
If you need to know details about the lookup, you're better off writing a script that properly implements the nuances of the DNS protocol, as that is not really the purpose of /dns. The fact remains, though, if your goal is to find out if a domain "exists" (is registered), DNS cannot reliably help; use WHOIS. On the other hand, if "exists" to you means "is not NXDOMAIN", realize that it's not a very meaningful distinction.
* edit: you don't even have to write a script, since
one exists
