#include <iostream>

class Base {
    public: virtual ~Base() { }
};

class Derived : public Base {
    public: int field;
};

void test(Base *ptr) {
    Derived *d = dynamic_cast<Derived *>(ptr);

    if (d) {
        std::cout << "field: " << d->field << std::endl;
    } else {
        std::cout << "Not a Derived object" << std::endl;
    }
}

int main() {
    Base b;
    Derived d;

    test(&b);
    test(&d);
}


-----------------------------------------------------------

$ g++ -fno-rtti test.cpp -o test
test.cpp: In function ‘void test(Base*)’:
test.cpp:12:42: error: ‘dynamic_cast’ not permitted with -fno-rtti
  Derived *d = dynamic_cast<Derived *>(ptr);