How can I check to make sure my subdomain is working?

NOTE:

It can take up to 24 to 48 hours for the subdomain to work properly so give it a few days.

Other than that, you can use this tool:

http://www.whatsmydns.net/

Once there, enter your subdomain into the search field, select "CNAME" in the drop-down menu next to it, and click Search.

Your results should point to Vyper: