[bredman]

In the context of full web pages how I've seen this done is by embedding a Javascript call that occurs after the page is loaded to your site. I suspect you could do something similar with this resource.

Alternatively you can look at what CDNs are hosting the resources and then just use the existing tools to compare the expected performance of each CDN. Of course this comes with a few caveats:

1/ Performance may be different on a given CDN depending on which "package" the customer has purchased. I know was the case ~1-2 years ago for Akamai.

2/ A CDN may perform well for small objects but not for larger objects, make sure you look at representative benchmarks.

3/ JS CDNs using CDN load balancing services like Cedexis mean more work to find all the CDNs that are being load balanced across (jsDelivr uses Cedexis)

(edit: formatting)