Checking whether the fuel filter is clogged usually requires a combination of symptom observation and actual inspection methods. Here are some commonly used judgment methods:

Observe the symptoms of vehicle operation (indirect judgment):
Engine weakness and poor acceleration: After the fuel filter is clogged, the fuel flow is blocked, resulting in insufficient fuel supply to the engine, especially when high throttle is needed (such as rapid acceleration or climbing), the symptoms will be more obvious, and the car will feel "dull".
Unstable idle or prone to stalling: Poor fuel supply may cause the mixture to be too lean, resulting in idle shaking and even stalling.
Difficulty starting: Especially during cold start, the fuel pump requires greater pressure and flow to establish initial fuel supply. If the filter is clogged, it may result in longer starting time or inability to start.
Increased fuel consumption: In order to compensate for the insufficient fuel supply, the engine control unit (ECU) may attempt to adjust the fuel injection strategy, but this may result in insufficient combustion and instead increase fuel consumption (this situation is relatively rare, more often due to insufficient fuel supply leading to a decrease in power).
Engine malfunction light on: ECU detects abnormal (lean) mixture ratio feedback from oxygen sensors and other sensors, which may trigger the malfunction light and store relevant fault codes.

Check fuel pressure (more accurate method):
This is the most direct way to determine whether the fuel system pressure meets the standard. It is necessary to use a fuel pressure gauge and connect it to the testing interface of the fuel pipeline (please refer to the vehicle maintenance manual for specific location and standard pressure values).
Start the engine and observe the reading on the fuel pressure gauge. If the pressure is much lower than the standard value and there is a large fluctuation or continuous decrease in pressure during engine operation, it is likely that the fuel filter is clogged (or it may be a problem with the fuel pump or fuel pressure regulator, which needs further investigation).
Attention: When operating the fuel system pressure detection, it is necessary to comply with safety regulations, release system pressure, and prevent fuel splashing.

Visual inspection of fuel filter (if allowed by design):
Some vehicles are designed with fuel filters located in easily accessible positions (such as inside the engine compartment), and the filters themselves have visible windows or can be removed from the casing.
If the filter can be removed, observe its inlet and outlet ports. If the color of the filter element at the oil outlet is abnormally dark, or if it feels very hard when gently pressed by hand, it may indicate severe blockage.
A more intuitive method is to remove the filter and try blowing or sucking in the opposite direction of the normal fuel flow, while ensuring safety (power off, pressure relief). If the resistance is very high and it is almost impossible to ventilate, it indicates severe blockage. Attention: This method requires great caution to avoid fuel injection and can only be used as an auxiliary judgment, not completely relied upon.
Read fault code:
Connect the diagnostic tool to the OBD-II interface of the vehicle and read the fault codes of the engine system. Although it may not directly display 'fuel filter clogged', it is possible to read fault codes related to lean mixture (such as P0171, P0174), which indirectly indicates that there may be a problem with fuel supply.

Summary and Suggestions:
The most common criteria for judgment are vehicle operating symptoms, especially decreased power and difficulty starting.
Fuel pressure testing is the most reliable diagnostic method, but it requires professional tools.
Visual inspection and reverse blowing/suction are relatively simple methods, but have significant limitations and require caution during operation.
If you suspect that the fuel filter is clogged, the safest method is to follow the recommendations in the vehicle maintenance manual, directly replace the fuel filter with a new one, and observe whether the symptoms improve. Because the fuel filter itself is not expensive and is a consumable, regular replacement is the best way to prevent clogging.
If you are unsure how to operate or judge, it is recommended to take the vehicle to a professional repair shop for inspection and replacement.