Checking if a class has expected attributes

I am going to give some classes about C++ and data structures, and to check students' progress I'd like them to develop the structures I talk about. This is the common approach for data structures classes, I guess. But I want more, I want the students to have a quick feedback on what they are missing, so I developed several unit tests for the classes that check the behavior and give them instant results on what is wrong.

This has been working properly for the past two semesters, but I want a step further on that automatized correction. I've been studying how to check what are the internal components of a class, so I can know if someone has implemented correctly a tree with a node* root and size_t size and hasn't used additional not-necessary attributes, for instance.

I know that I can have a rough approximation of an object size with sizeof, but the results are not that precise. It frequently is different from what I expect, for example: I tested creating a class with a pointer (8 bytes) and an int (4 bytes), but the sizeof was 28. From what I learnt, probably this has something to do with virtual function table and other alignment stuff.

So, how far and further can I go analyzing if someone has coded a data structure the proper and expected manner? How can I check that someone just didn't #include <list> and created an adaptor (for this I know I can just strip the includes but anyway)?