Hi Rizal,
The easiest way to do that would be to gather the output of 'show access-list' now. Then, in one month, get the output again. Compare the two files and any lines that haven't seen an increase in the hitcnt field have not been used (especially if they are 0).
Keep in mind, these values aren't saved across a reload so if the firewall reboots you'll have to start over.
Hope that helps.
-Mike