Analysis of the oil should be performed every third or fourth oil change to get an indication of pending problems. This time interval is quite nebulous. If your fuel is kept clean, the filter doesn't get dirty. Check the filter. Unfortunately, to check screw-on type filters they must be destroyed.
Most primary filters have removable cartridges that can be inspected.
If any water is evident, it's time to get water off the bottom of the tank. Racor has a couple of interesting devices. One is a vacuum gage kit that you see into the suction side of your fuel lift pump. The gauge mounts anywhere and if it stays near zero, the filter is clean. As soon as vacuum increases, it's time for a filter change and investigation into the source of the contamination.