Poor performance or loss of power
slow to rev or decrease in RPMs
Low fuel pressure
Check the plugs and see what color they are. They should be tan/brown if the engine is running well, black if it is too rich, and, what you will be looking for if the fuel filter is very clogged, white.
The best way to determine is to run a fuel pressure test. But that is often just as time consuming as replacing a filter in most cars.