The clearest sign that your fuel filter is clogged is an appreciable loss of power when accelerating, especially from a complete stop or attempting to drive up a steep incline, such as a bridge.
A fuel filter protects your engine by capturing small particles that get into gasoline during the transportation process. Over time, those particles cover the surface of the filter material and eventually prevent the free flow of gasoline (or diesel) from the fuel tank to the engine. Filters should be changed between 40,000 and 60,000 miles, depending on the vehicle manufacturer's recommended schedule. Difficulty starting a vehicle is not a sign of a clogged fuel filter, since the amount of fuel needed to start a vehicle at idle is minimal.