Check length of textfield in header file
This should do it for a text field:

craft.entries.section('aboutus').textField(':empty:')|length


To see if a Matrix field has any blocks, it's not as pretty.  There's probably a better way, but it's all I can think of at the moment:

{% set entries = craft.entries.section('news') %}

{% set validEntries = [] %}

{% for entry in entries %}
    {% if entry.matrixField|length %}
        {% set validEntries = valid|merge([entry]) %}
    {% endif %}
{% endfor %}


validEntries is now an array of entries that have at least one block.