Just do expression SFINAE and discard the result:
template <typename C, typename... Args>
struct is_call_possible {
private:
template<typename T>
static auto check(int)
-> decltype( std::declval<T>().operator()(std::declval<Args>()...),
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
// overload is removed if this expression is ill-formed
std::true_type() );
template<typename>
static std::false_type check(...);
public:
static constexpr bool value = decltype(check<C>(0))::value;
};