it should report the external dns caching server been used yes.
So if it reported a internal ip that would be strange.
In my case, my phone uses my router as its resolver so 192.168.1.253. The router sends uncached queries to its upstream server I configured, which is a personal DNSCrypt, DNSSEC dual stacked, no logging, no neg caching server I run myself for personal use at a UK datacentre, and its that UK ip that gets reported by dns leak tests, and as such I know if the configuration is working properly. I also have the dnscrypto.org .fr DNS server as a upstream DNS server, but its only a backup if my personal server goes down.
I am considering moving my DNS server out of the UK tho, as I wonder how much power the UK authority has in been able to get the transit provider at my UK location to sniff my dns traffic, the DNScrypt will encrypt between my router and the server which stops sky sniffing, but not from the server to other DNS servers. The problem been that DNScrypt is a hack, its not a standard, so there is no encryption protocol for server to server queries.
Nslookup I would expect to report 192.168.1.253 as the resolver as it only knows the immediate resolver its directly querying.