To my knowledge, there isnt a direct way to validate SOAP Schema in Postman. It’s utilized more around JSON services, so you won’t see many things around XML.
However, since Postman is built off node.js, you might be able to use a node.js library thats not included in Postman. Below is a node.js library that could be used to validate xml schema. I have not personally used it, but I searched npm and its what I found.
I have also not tried to use different external libraries that are not within Postman, so I am not sure how this would work (or even it if would work), but figured I’d throw around the idea.