how to monitor mailscanner is running

[barry]

A busy server which I administer stop running MailScanner suddenly two
days ago. The sendmail was still running and receiving email. Only the
MailScanner  stop running because I found that there is no mailscanner
header in all email received.  There were no error messages when I
restarted the MailScanner services. I can still see the Mailscanner
process is running. But the MailScanner still not function as there
still no mailscanner header in all new email received. MailScanner can
only work again until I kill all running sendmail processes and
restarted the MailScanner again.
My question is how can I monitor MailScanner is running and doing its
function.