example.cpp

template <class OutputIter, class = void>
class Foo;

template <class OutputIter>
class Foo<OutputIter, is_output_iterator<OutputIter>> {
    /* some class with an output iterator */
};

output_iterator_check.cpp

#include <iterator>
#include <type_traits>

template <class Iter, class Tag>
using has_tag =
    std::enable_if<
        std::is_convertible<
            typename std::iterator_traits<Iter>::iterator_category,
            Tag>::value,
        Iter>;
 
template <typename Iter>
using is_output_iterator = has_tag<Iter, std::output_iterator_tag>;