I think there are different scenarios that a DNS test should cover:
change from a specified value
any change, e.g. TTL, or change of the resolved value
But the URL method mentioned by @Felix to test DNS records is also very creative way to tacle this.
@Dean Keep in mind that the TTL may be the issue here as those cached records will continue to resolve, while the TTL is still valid. e.g. 24 hour TTL. The more important test here that you are describing is the DNS server is reachable, not the DNS records. If you can’t device a DNS reachability test, you could alternatively set a dummy A record with 5 second TTL. This might solve, if you cannot test your DNS server is reachable.