[exim] Detecting a deferred sender callout verification

I am using ACLs to store the "OK" result of a sender callout
verification in an ACL variable:

  warn   verify               =
              sender/callout=30s,connect=10s(,defer_ok)/no_details
         set acl_m_callout_ok = true

so that I can later skip certain tests (e.g. deny !verify =
reverse_host_lookup)
if callout verification succeeded.

But I would like to know also if callout verification resulted in a
defer, so
that I could for instance do a defer !verify = reverse_host_lookup in
such
a case.

Question: Is it somehow possible to find out in a condition check
whether the
last sender verify callout resulted in a :defer:?