Besides the flight number, another way is to glance in the System Timetable (download at NWA.com). If the equipment is shown as 320, DC9, etc, it is a NW operated flight. If "AIR" is shown, it is a codeshare flight. Looking at the current timetable for MSP-ATL shows several DL flights numbered #89xx with AIR, and several NW flights with #1500 or less and specific aircraft identified.