[Exim] Conditional drive me nuts!

OK - I need a little help here. Trying to make spam assassin work and I
need a conditional and the language drives me nuts.

I need a something like this

condition = "${if {def:h_X-Spam-Status:}{no}{yes}}"

but that doesn't work.

I want a condition that will allow me to test if the message has already
been touched by spam assassin and return true if it has not.

I think this is the last piece I need to make this work.

Thanks in advance.