Akamai Whoami Service
The Akamai whoami service can be quite helpful for troubleshooting, especially since it was updated to support ECS.
Here is an example dig using Cloudflare's resolver service.
$ dig @1.1 +short TXT whoami.ds.akahelp.net "ns" "172.69.162.71"
As you can see the results contains NS, being the IP that the resolving nameserver sourced it's query from to Akamai.
Cloudflare does not send the EDNS Client Subnet Header to authoritative servers, so no other information is mapped other than the resolvers IP.
Here is a dig using the Google resolver.
$ dig @8.8.8.8 +short TXT whoami.ds.akahelp.net "ecs" "61.245.0.0/24/24" "ip" "61.245.0.100" "ns" "172.253.204.3"
Because Google supports ECS you can see in this case, in addition to the source of the resolving nameserevr, the remote authoritative DNS server is able to determine the client IP and subnet where the query was originally sourced. This information is passed on by Google's resolving DNS when they query is made.
Some CDN's use this information to map users to their closest CDN node so that content can be served fast to end users. However not all public DNS resolvers support this feature, indeed many standard ISP resolvers don't either, so it's not necessarily a method that CDNs can rely on to map users. Although if you get at least the NS record response as in the Cloudflare example you should expect to get reasonably close guess as to the location of the end user.