Once you have set-up you CNAME record following the directions outlined here you can check that the configuration is correct using a DNS look-up tool.
Step 1: Proceed to DNS lookup tool here
Step 2: Enter your mapped domain i.e returns.testingstore.com
If configured correctly you will see a CNAME record that points to domains.returnly.com
If configured incorrectly you will not see a CNAME record or it will point to an incorrect domain URL.