In the mgmt web interface on the ScreenOS firewall you will want to look at the logs for the trust to untrust policy.
Menu: Policy > Policies
on the list find the trust to untrust section
In the middle options column click on the grid icon to open up the logs with traffic details
If the grid icon is not desplayed, edit the policy using the button nearer the right side and turn on logging by checking the box. This will start collecting logs then going forward.