how to determine if object element is unsigned or signed integer after casting

how to implement function to check if the original value is negative and not unsigned, then after the casting it changes its value to uint.

given:

struct Object
{
    uint64_t a;
    string b;
}

class myclass
{
public:
   bool function(const object& obj);
};

bool myclass::function(const object& obj)
{
**// if obj.a is negative number return false**
}

int main()
{
 object ob;
ob.a = -5;
ob.b = "fdfdssdgdf";
myclass m;
bool boo = m.function(ob);
if(boo == false)
 cout << "GOOD" << endl;

return 0; 
}