[exim] how to detect if address aliased?

Sorry the quick'n'dumb question.
In short, how do I detect that a redirect router has successfully
redirected an address?

I only accept a local recipient if it is an alias, this is because of the
way I handle virtual domains. Each domain has got its alias file (plus a
server wide alias file for role addresses) and these are checked by three
routers. One for the global aliases, one for the prefixed aliases (the
suffix is not dropped when looking up the local part) and the last one for
unprefixed aliases (the suffix is dropped). When I finally arrive at the
local user router I want to check that the address has been aliased, as
I do not allow delivery otherwise, how do I check that?

Thanks

Giuliano