Chrome devtools network tab shows "Transferred data over network" value of the same size of cached resource file. If I block the last request that appears on above image, picture isn't loaded, but the second request still shows the same amount of data was transferred. How to know if the image resource is really being pulled from cache and bandwidth is being saved?
Note: I'm using max-age Cache-control policy and in Firefox it is clearly working.
The ultimate answer to you question can be provided by the access logs of the server you are requesting the ressource from. Just add an "fingerprint parameter" to your request and check the access log from the server.
Another option would be using an extension like webQsse (https://chrome.google.com/webstore/detail/webqsee-web-sniffer-recor/gamdpfnfkjknkimfbboonmgdfnondfme) which also displays where the image is loaded from.