Test value of virtual docfield in a unit test

I understand that values of virtual docfields are calculated when a document is presented. If I want to verify that the values computed by virtual docfields are correct in a unit test, how would I be able to do this?

I think I found out the solution to this by calling the get_valid_dict() method of the Document object and then assert the values of virtual doc fields from the returned dict. Although, there is an as_dict method too, which appears to call get_valid_dict() method. Which of the two is the right method to use?