We have used both self-hosted coturn and a major vendor's managed TURN service, and we found that neither provides a way to review in the TURN server whether it received a request from our test page, or if it did, what it did in response.
Two bits of advice that have been helpful:
1) If you can arrange to use a Linux device at the client side, set its OS-level TTL property above 70. This determines how many router hops you are asking routers to permit enroute to the TURN server. We found this needs to be higher in the EU than the US, but it's not controllable from the browser's js api and merely helps reveal if the problem is due to hop count. If this is the problem, try to put one of your service endpoints nearer to the users in this location.
2) Some TURN servers seem to ignore options in your iceServers beyond a certain number, seemingly beyond four. So be sure that your preferred options are among the first four. It might even help to delete all but 1-4.