How to recognise messages from local network or authenticated connection

I am running sendmail with procmail, which feeds the messages to
spamassassin.  I do not want to run local or messages from
authenticated connections through spamassassin.  What is the best way
of recognising these headers in procmail?  At the moment I'm using the
"Received: from " header, but there are a lot of different
permutations and combinations of these and the best I can do at the
moment is to use the name of every sending computer on the local
network.

Is there a straightforward and robust way of doing this?

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