Find out if any loop content in array from outside array?

Hi, does anyone know how / if you can find out if an array has any loop content from outside it? So for example (this doesn't work):

{% if page.header.page_nav.count > 0 %}
do something
{% endif %}

I can't do it from inside the loop otherwise this would work:

{% for item in page.header.page_nav %}
{% if loop.first %}
do something
{% endif %}
{% endfor %}

.count does not exist. use the length filter: http://twig.sensiolabs.org/doc/filters/length.html

Ok thanks