How do I determine the presence of a using/typedef in C++?

See more:

C++14

Just to note, I cannot use the STL for this because one of the environments I'm targeting does not have a complete or accurate implementation of it.

I think I can use SFINAE to do this, but as much as it has been explained to me, I can't seem to wrap my head around it.

I have a type say:

struct Foo {
    using test = char;
};

and

struct Bar {
     // empty
};

I'd like to design a template type or template function such that it can detect the presence of the "test" member.

I know this is possible to do at compile time, again probably using SFINAE.

What I have tried:

I have a less elegant way to get what I want, but it's a lot of code to show here, and would require some explanation because it's pretty abstract.

The result I think would look something like this?

template<typename T> static inline constexpr bool check_test(typename T::test value) {
   return true;
}
template<typename T> static inline constexpr bool check_test() {
   return false;
}

That's not right, but i don't know how to make it right. I want to check if test is declared on the class/struct.