[exim] check for a header value in a router

Hello list,

I'm a bit new to exim, and ran into a problem. I've created a router,
which, if the conditions match, will add a header to the incoming mails.
In the next router I want to check for this header in the conditions
row. Is it possible to do it within exim, or do I have to call an
external program for it?

Regards,
Gergely POLONKAI