On my fuel filter with the water separator there is a knob at the bottom you can turn to drain it. Take a small container and drain some fuel and see if there is water in it. On my other filter there is a ~clear plastic bottom, I take a flashlight and see if I can see any Algae or crude in it.
I had algae one time when I first bought the coach, a black slimy/stringy type build up, it almost choked the filter closed. I now use Biobor Algaecide if the rig is going to be parked any length of time. I also keep the tank full to reduce condensation.
2002 Monaco Windsor 38 PKD
2005 Jeep Grand Cherokee w/5.7 Hemi