The only way to tell for sure is to remove the filter and blow through it. If there's little resistance, the filter is still okay and does not need to be replaced. But if there's more than minimal resistance, the filter is dirty and should be replaced.
CAUTION: Gasoline is poisonous, does not taste very good and may burn sensitive lips. So don't hold the filter to your mouth to blow through it. Instead, attach a short piece of clean rubber hose to the filter and then blow through the hose to test the filter.
A completely plugged fuel filter will stop your engine cold by choking off the flow of fuel to the carburetor or injectors. The engine may not start, or it may start, then stall and die.
Some filters have a spring-loaded bypass, however, that allows fuel to bypass the filter element if it becomes clogged. Fuel continues to flow, but it may carry dirt to the carburetor or injectors, which can create additional problems.
A partially restricted filter will usually pass enough fuel to keep the engine running at idle or low speed, but may starve the engine for fuel at higher speeds or loads. So your engine may run fine putting around town, but sputter and lack power when you try to drive at highway speeds or pass someone.
Located inside the fuel tank is a screen or mesh sock that acts like a prefilter to keep big pieces of dirt and rust from being drawn into the fuel pickup tube or tank-mounted electric fuel pump. If the screen becomes clogged with debris, it can have the same effect as a plugged or dirty fuel filter. Therefore, if you've been experiencing a fuel starvation problem and have replaced the fuel filter -- and it didn't help -- the screen in the tank is probably the culprit. To clean or replace it, the fuel tank usually has to be removed.
WARNING: The fuel tank must be drained prior to removal. The fuel must be stored in a sealed "approved" container. The battery should also be disconnected to prevent any accidental sparks from an in-tank electric fuel pump connection from igniting the vapors. Do not smoke when working on the fuel tank, filter or fuel lines, and keep all other sources of ignition away (electric heaters, pilot lights, etc.) from the work area.
Replacing the fuel filter periodically (every year or so) for preventative maintenance can reduce the risk of filter-related driveability problems. Most vehicle manufacturers, however, no longer specify a replacement interval for the fuel filter. Or, if they do it's some incredibly long interval like once every five years or 50,000 miles. Many mechanics feel this is unrealistic. Waiting that long to change the filter is asking for trouble, especially if you drive on gravel or dirt roads, buy the cheapest gas you can find from "cut-rate" stations, use gas with alcohol in it, or your vehicle is more than six or seven years old and may have rust in the tank.
The fuel filter on carbureted engines is usually located at the inlet fitting of the carburetor, or an "in-line" filter is used between the fuel pump and carburetor.
When replacing a filter that screws into the inlet fitting on the carburetor, be careful not to overtighten the filter. The threads in the carburetor are relatively soft and can be easily stripped. But also make sure the filter is snug so that it doesn't leak. It's okay to apply some gasket sealer to the filter threads to assure a leak-free connection. But do not use RTV silicone sealer (which gasoline dissolves) or teflon tape (pieces of which can flake loose and end up in the carburetor).
When replacing an in-line filter, most filters come with two new rubber hoses that go on either side of the filter. Use them. Don't reuse the old hoses because rubber hoses deteriorate over time and can leak or shed small flakes or rubber that can end up in the filter or carburetor. Also, make sure the hose clamps are properly positioned and tight.
NOTE: Most in-line filters have an arrow showing the direction fuel should flow through the filter. Install the filter so the arrow points toward the carburetor.
Fuel filters on fuel injected engines are usually larger and have a finer filter element than those on carbureted engines. Consequently, they are usually more expensive.
The filter may be located anywhere between the fuel tank and injector fuel supply rail or throttle body. On many cars, light trucks and minivans, the filter is located underneath the vehicle along a frame rail. On some, the filter is part of the electric fuel pump assembly inside the fuel tank! Refer to a shop manual for your fuel filter's location.
CAUTION: Fuel injected engines usually have a lot of residual pressure in the fuel line, even when a vehicle has sat overnight. So either follow the manufacturer's recommended procedure for relieving pressure in the line prior to removing the filter (applying vacuum to the fuel pressure regulator manifold fitting, or cranking the engine with the ignition disabled), or wrap a rag around the hose connections and slowly loosen them.
If the filter has an arrow indicating the direction of flow, it should be installed with the arrow pointing toward the engine and away from the fuel tank.
If the filter is located inside the tank, the tank will probably have to be removed. Follow the same precautions as previously described for replacing a plugged pickup screen.