I found the answer to my own question.
You can turn on mod_rewrite logs and depending on loglevel it can log all the rewrite rule actions.
Create a logfile "myrewrite.log" in your apache logs directory. Add these lines right before your Rewrite rule:
RewriteEngine on
RewriteLog "/usr/local/apache/logs/myrewrite.log"
RewriteLogLevel 9
it's explained in mode_rewrite reference documentation:
http://apache.rcbowen.com/httpd-docs-1. ... RewriteLog