The best way to test your site would be to tell your OS to use the TCH DNS servers to resolve. Since the servers at TCH are already set up the way the rest of us will see the DNS entries once you make the change with your registrar, directly accessing them will make it look like you've already mad the chage. N.B. depending on the TCH server configuration, you may not be able to browse any sites that they don't host, and it isn't very polite to use their DNS servers for recursive queries on a regular basis.
If you're using a Windows OS, the DNS server setting is in a different place depending on the version, and type of connection to the Internet you have. In all cases you'll be looking under TCP/IP properties, and selecting 'Specify DNS servers' (or equivalent). just enter the IP address of the SNH servers (which I'm too lazy to look up right now).
DON'T FORGET TO MAKE A NOTE OF THE WAY IT IS SET UP BEFORE MAKING THE CHANGES.
You'll have to change it back after you're done testing.