I have a class that serializes a set of objects (using XML serialization) that I want to unit test.
My problem is it feels like I will be testing the .NET implementation
Seeing how you can't really fix serialization, you shouldn't be testing it - instead, you should be testing your own code and the way it interacts with the serialization mechanism. For example, you might need to unit-test the structure of the data you're serializing to make sure that no-one accidentally changes a field or something.
Speaking of which, I have recently adopted a practice where I check such things at compile-time rather than during execution of unit tests. It's a bit tedious, but I have a component that can traverse the AST, and then I can read it in a T4 template and write lots of #error
messages if I meet something that shouldn't be there.