Is an is_functor C++ trait class possible?

How can I deduce statically if an argument is a C++ function object (functor)?

template <typename F>
void test(F f) {}

I tried is_function<F>::value, but this doesn't work. It also seems there is no is_functor trait, so perhaps it's not possible. I appear to be only looking for a specific member function, in this case the function call operator: F::operator().