Because category headers and footers are rendered via an SMT Item, there isn't a variable to detect before that item is rendered. So, the solution is thus:

<mvt:capture variable="g.has_header">
<mvt:item name="prod_ctgy_hdft" param="ctgy_header" />
</mvt:capture>

Now you have a variable you can use:

<mvt:if expr="g.has_header"> do something </mvt:if>

Obviously, this can be used for any SMT item that renders content.