Allow debugging all policies, in order to remove ports included in them, which are not really traffic used and could be removed from the policy.
I'd suggest checking the policy hits/ top policies but sorted upside down in the WebUI. This can show you the least used policies.
Policy map in dimension also shows policies via most/least used.
WatchGuard Customer Support
i think question is not to know if a policy is used or not (even if it's just useful also according to me..) but to identify any un-used port defined in a policy ;-))