DNS packages doesn't contain information about its source and destination they are doing automatically using the DNS query cascade.
What you can do to know if it's working the forwarders or not is to set up a client with the Windows Server DNS IP as only DNS.
Make sure to clean up the cache by executing (ipconfig /flushdns) on client.
Then try without any forwarder configured it, you should have issues to hit external sites. (remember that there's cache information involved in client and in server too that needs to be cleaned up).
Then try using one forwarder like 188.8.131.52 and you should be able to reach the sites, but the specific ip of what DNS server is being queried you won't be able to get that information since in the networking level there's no such information.
Here's an interesting question related to what you want: https://www.experts-exchange.com/questions/24079211/How-can-I-trace-the-DNS-forwarder-query-from-my-Server.html