What's the best way to setup a conditional to see if a matrix block is used?
I would use a loop to set a conditional, and then check it afterwards. You need to initialize the conditional outside of the loop, otherwise it won't be set outside the loop later.

{% set haveAVideo = false %}
{% for block in matrixBlocks %}
  {% if block.type == 'video' %}
    {% set haveAVideo = true %}
  {% endif %}
{% endfor %}

{% if haveAVideo %}
  {# video block js #}
  {% includeJsFile ... %}
{% endif %}