Check if variable is string or array in Twig

Can be done with the test iterable, added in twig1.7, as Wouter J stated in the comment :

{# evaluates to true if the users variable is iterable #}
{% if users is iterable %}
    {% for user in users %}
        Hello {{ user }}!
    {% endfor %}
{% else %}
    {# users is probably a string #}
    Hello {{ users }}!
{% endif %}

Reference : iterable