Based on the poster's phrasing, it's more than just testing, IMO. After you write your unit test for the API and make sure everything is working as expected, you need to monitor third-party APIs so you catch issues before the users do. That's the real risk with third-party APIs - it's not your code and you have no control over how much testing was done on the API or when/if it changes.
(Disclaimer: product names used here) If you use soapUI to write your API tests, those tests can be re-used in AlertSite as an operational monitor to make sure the API keeps working as expected. If it fails the test, you can get alerted before your users call you and complain that your app doesn't work.