Run-time check of integral conversion

The best I can imagine, in a simple way, is check if T(u) == u and the signs of u and T(u) are the same

I mean something as

template <typename T, typename U>
bool IsRepresentable (U const & u)
 { return (T(u) == u) && (T(u) > T(0)) == (u > U(0)); }