[floatboth]

With some forms of caching it's much simpler: the browser sends an ETag or If-Modified-Since and the server is supposed to return 304 Not Modified to optimize the load if the cached resource is still valid.

[kortilla]

But from JavaScript I don’t think you can see that. You just get the end result of the image being served to you. You have to infer it from timing.

[megous]

https://megous.com/dl/tmp/705dc9a2477d1f95.png

For cross-origin, you'd add CORS.