I think FluentValidation is a great way to write validation code and is really readable. What is also great is the unit testing support you get out of the box.
This really is one of those times where you need to write breaking tests first, proper TDD (not DDT – which is so often what I come across!). Fluent doesn’t just run the validation of the property you specify in your assertion, all properties are validated, so pass in a valid object with only the one property changed that you want to test. I’ve also got in the habit of always having one test that tests that your valid object is valid.
Example code (I can’t guarantee this will compile, I’ve santitised it since last building) :