Use SFINAE to detect whether to_string is present.

sfinae.cc

#include<string>
#include<iostream>
#include<utility>
#include<type_traits>

template <typename T, typename TType, bool value =
    std::is_same<std::string, decltype(to_string(std::forward<T>(*(T *)nullptr)))>::value
>
struct has_to_string { };

template <typename T, typename TType>
struct has_to_string<T, TType, true>
{
    typedef TType type;
};

template <typename T, typename TType = void>
using has_to_string_t = typename has_to_string<T, TType>::type;

template <typename T>
has_to_string_t<T, std::ostream &> operator << (std::ostream &ost, T &&value)
{
    return ost << to_string(std::forward<T>(value));
}

struct T1 { };
struct T2 { };

std::string to_string(T1)
{
    return "T1";
}

namespace NS
{
    struct T3 { };
    std::string to_string(T3)
    {
        return "T3";
    }
}

int main()
{
    // The following wouldn't work because
    // ADL won't find std::to_string.
    // This is fine since std::to_string only
    // deals with some basic types, which are
    // already covered by std::operator <<.
    // ::operator << <int>(std::cout, 1) << std::endl;
    std::cout << 1 << std::endl;
    std::cout << T1() << std::endl;
    // Does not compile:
    // std::cout << T2() << std::endl;
    std::cout << NS::T3() << std::endl;
}