To determine whether a filter (including air filter, oil filter, fuel filter, etc.) needs to be replaced, observation and inspection can be carried out through the following aspects:

1、 Follow the maintenance cycle (the most basic method):
Refer to the owner's manual: This is the most authoritative basis. Manufacturers will provide replacement mileage or time intervals for air filters, oil filters, fuel filters, etc. based on the normal usage of the vehicle (e.g. every 5000 kilometers/6 months, or every 10000 kilometers/1 year, etc.).
Regular maintenance: Even if the filter seems to be functional, it is recommended to replace it according to the manufacturer's recommended maintenance cycle. This is preventive maintenance that can maximize engine performance and lifespan.

2、 Observe the condition of the filter itself (applicable to removable filters):
Air filter:
Visual inspection: Remove the filter element and look into the light. If the surface of the filter element (especially the windward side) accumulates thick dust, or the filter material becomes very dirty or discolored (such as turning black), it is recommended to replace it even if it is not yet the replacement cycle. Especially for paper air filters, once they are damaged or severely clogged, they must be replaced.
Touch check: If it is a washable oil bath filter, check whether the filtered oil is uniform, whether it has been severely missing or deteriorated.
Oil filter:
Visual inspection (relatively difficult): Oil filters are usually made of metal shells, and the internal filter element is not easy to observe directly. But you can check whether the shell is corroded or cracked, and whether there is leakage in the installation base. If the oil pressure is found to be abnormally low, it may also be a sign of filter blockage (but this situation is often already quite serious).
Combined with oil change: It is usually recommended to replace the oil filter at the same time as the oil, as the old filter will leave a large amount of old oil and impurities, which will affect the cleaning effect of the new oil.
Fuel filter:
Visual inspection: Similarly, it is often difficult to directly observe the interior. You can check whether the shell is damaged or leaking.
Performance correlation: If the vehicle experiences symptoms such as weak acceleration, unstable idle, difficulty starting, and increased fuel consumption, and other possible causes are ruled out (such as spark plug and fuel injector problems), it is possible that the fuel filter is clogged, resulting in poor fuel supply.

3、 Observe the vehicle's operating status and performance changes:
Engine performance degradation:
Weak and sluggish acceleration: A clogged air filter can limit the intake air flow, resulting in an overly rich mixture or insufficient combustion, manifested as weak acceleration. A clogged fuel filter can also lead to insufficient fuel supply, resulting in weak acceleration.
Unstable idling and shaking: Uneven intake or fuel supply may cause unstable idling.
Increased fuel consumption: Air filter blockage leading to poor intake, or fuel filter blockage leading to poor fuel supply, can both affect combustion efficiency and result in increased fuel consumption.
Difficulty starting: When the fuel filter is severely clogged, it may not be able to provide sufficient fuel pressure, resulting in difficulty starting.
Abnormal exhaust: Severe blockage of the air filter may cause the mixture to be too rich and the exhaust to emit black smoke. Oil filter issues may cause oil to enter the combustion chamber and emit blue smoke (but this is more related to piston rings, valve seals, etc.).
Increased engine noise: In some cases, clogged filters may cause abnormal engine operation and produce abnormal noises.
Engine malfunction light on: Although there are many reasons for the malfunction light to light up, if it is accompanied by the above performance problems and the filter is approaching the replacement cycle recently, replacing the filter (especially the air filter) can sometimes solve the problem.

4、 Consider the usage environment of the vehicle:
Harsh environment: If the vehicle frequently travels on dusty construction sites, gravel roads, coastal areas (salt spray corrosion), or cities with poor air quality, the burden on the filter (especially the air filter) will increase, and it may need to be inspected and replaced more frequently than usual.
Summary and judgment points:
Primary basis: Strictly follow the maintenance cycle in the owner's manual.
Visual inspection: Regularly remove the detachable filter and observe its degree of dirt and damage.
Performance correlation: Pay attention to whether the vehicle has abnormal symptoms such as weak acceleration, increased fuel consumption, and difficulty starting.
Environmental factors: When used in harsh environments, the replacement cycle should be appropriately shortened.
If the judgment result is uncertain or there are obvious abnormalities in the vehicle, it is best to have a professional maintenance technician inspect and replace it. Don't compromise the engine's lifespan just to save money.