check inheritance of underlying type of 2 variables - real types of both are incomplete type

I am learning C++ about incomplete-type.
I think this is impossible, but I am not sure.

Question:-

Is it possible to check inheritance between underlying type of 2 variables if both are incomplete types?

Test1.h:-

#include <vector>
class A{};
std::vector<A*> test1();

Test1.cpp:-

#include "Test1.h"
class B : public A{};
class C : public B{};
class D : public A{};
std::vector<A*> test1(){
    A* b=new B();     A* c=new C();     A* d=new D();
    std::vector<A*> re;  
    re.push_back(b);  re.push_back(c);  re.push_back(d);
    return re;
}

Test2.cpp:-

#include "Test1.h"
int test2(){
    std::vector<A*> v=test1();
    A* b=v[0];  A* c=v[1];  A* d=v[2];
    isInherit(b,c); //true
    isInherit(c,d); //false
    isInherit(b,d); //false
}

If the answer is yes, please show how to code bool isInherit(A*,A*).

Note that modifying class A is not allowed.
(Thus, a hack to set a non-static field of A into a variable by-type is not allowed.)

I may use it as some kinds of black-magic inside my code.