In C, should Unit Test be written for Header file or C file

I have a header file which contains declaration of struct and some methods, and a 'C' file which defines(implements) the struct and the methods. Now while writing Unit Test Cases, I need to check if some struct variable(which do not have getter methods) are modified.Since the struct's definition is contained in the C file, should the unit test cases be based on header file or C file ?