Which version of Chrome are you using? The latest versions have become much better at reporting CORS issues. For example, I am using Chrome version “32.0.1700.14 beta”, and when I visit this page, I get the following error in my console:
Request header field X-Foo is not allowed by Access-Control-Allow-Headers.
This information is only available from the console, and cannot be programmatically accessed. Hopefully more browsers will follow suit. Note that if your request fails for some other, non-CORS reason, you may still see unhelpful error messages.