1. Use a multimeter to test the resistance value. Place a multimeter on the electrical barrier and test the resistance at the input and output of the surge protector. Generally, the resistance value of a surge protector should be very high. If the reading is lower than expected or appears to be infinite, it may be a sign that the surge protector is damaged.
2. Observe the appearance and color changes. Surge protectors usually have a fault indication window. Normally, this window appears green. If the window color changes to red, this usually means the surge protector has been damaged by a lightning strike or other high-voltage event and needs to be replaced.
3. Check lead contact and voltage drop. Measure the resistance between the surge protective device leads to make sure they are in proper working order. Also, check whether the voltage drop of the protector is within the rated range.

To sum up, the most direct way is to judge the status of the surge protector by observing the color of the indicator window and measuring resistance with a multimeter and professional testing are auxiliary means.
If none of the above methods can determine whether the surge protector is damaged, it is recommended to consult a professional or spd manufacturer for judgment.