If it looks like there is sensor attached near or at the bottom of the fuel filter housing or collection bowl, then you probably have a WIF (water-in-fuel) sensor which should alert you up at the driver's seat in some way. If no sensor, then you probably have to check periodically like others have mentioned.
'07 Rockwood 8281SS 5th Wheel
'07 Silverado 2500HD LMM Duramax Diesel / Allison 1000 CC 2LT S/B
Reese 15K 5th Wheel Hitch w/ Kwik-Slide