Is it possible to write a template that changes behavior depending on if a certain member function is defined on a class?

Here's a simple example of what I would want to write:

template
std::string optionalToString(T* obj)
{
    if (FUNCTION_EXISTS(T->toString))
        return obj->toString();
    else
        return "toString not defined";
}

So, if class T has toString() defined, then it uses it; otherwise, it doesn't. The magical part that I don't know how to do is the "FUNCTION_EXISTS" part.