How to identify an XXE vulnerability in a webserver?

Hey forum, just looking for some more detail on XXE vulnerabilities please.

When I capture a response from a webserver that looks like this:

<?xml version="1.0"?>1231232343home applicances17th Estate, CA

What is it that tells me that there is an XXE vulnerability on this system?

I know there is one because its on a starting point machine. But this response from the server (to me) looks like every other xml response that I have ever captured.

Appreciate your help…