[exim] Detecting presence of custom header in ACL

I have a hopefully simple question and for whatever reason I can't
seem to
find a way to get it to work. Long story short, in a given SMTP time
ACL,
as part of my final accept I might add a custom header for
debugging/tracking purposes. What I would like to do is detect if the
header already exists prior to adding it. I'm assuming I can use a
condition that detects if $h_X-HEADERNAME is present and if not
add_header.
I'd like to do this at ACL time rather than in the router or transport
using header_add or header_remove. Any thoughts on this would be
appreciated. I'm sure this is easy, I've heard of this looking for the
X-SPAM-STATUS header added by SA and if it detects an existing one, do
some
action.

Thanks,
James