VM:Secure doesn't interact with diagnose X'84' calls in any way. So there isn't any way to tell if this is in use other than watching for a failure.
However, you may want to cause this failure before encryption to find areas to check before hand. Another way to cause a failure before you encrypt the passwords is to change the privilege class for the diagnose so it is assigned a class alone, at least temporarily. It will fail due to not having the correct privilege class which could be easily fixed (add the class to the directory entry) but this allows you to trap the use so you can check what is using the diagnose it to see if password will be an issue.
Either way, it will be a failure that tells you something doesn't work. It may give you some advantage to find these issues before you encrypt the passwords, it may not.