Use only ns2 for a test domain and see if it resolves.
█ YagHost - Pure SSD Hosting | Since 2007 | Average Response Time: 15 min
█ Web Hosting | Reseller Hosting | Managed VPS Hosting
█ 99.9% Server Uptime Guarantee | 24/7 Rapid Response Tech Support | 30 Day Money Back Guarantee
█ LopHost.com - Web Hosting Tutorials
This may sound silly, but shutdown or disconnect NS1 briefly, and then test domain resolution (DNS) from another computer that hasn't recently cached the domain. If you get a DNS response, it must have come from NS2. I assume that NS1 and NS2 are not running on the same DNS server.
Note: I also assume that the DNS lookup wasn't cached upstream, either. Perhaps using a cellphone to check (via a separate network such as the cell network instead of wifi) would be a good way to test.