how to test procmail rules?

This may sound utterly silly, but within all these years, that I have been using procmail, I never came around to find a way of testing procmail rules other than by testing them in production for my incoming e-mail. The problem is, procmail gets called through sendmail and they deliver the messages. If I resend messages, sendmail changes the Return-Path header field, on which my rules are based. All other relevant looking header fields can get tweaked and faked, Return-Path seems alright (???).

I'll try to find out on the procmail list, ie. on http://dir.gmane.org/gmane.mail.procmail .

Update / 2010-12-26:

Setting "VERBOSE=on" within your .procmailrc shows a lot of helpful output.

Yes, a message, that my .procmailrc does not deal with, as I think it should, must get copied to another IMAP folder; from there I copy it back to the IMAP folder, which I direct fetchmail to. I can repeat that endlessly, until I find out what's going on. Maybe that also works somehow with formail, but I haven't figured that out yet.