Nowadays, I'm tried to use a TVS diode but I think it is damaged. How can I test a TVS diode? How can I understand that is it working or not? What is the best way to test TVS diodes?
For repairs, if it has a charred hole in the package or is in two pieces it has failed open. If it measured shorted it is bad. Otherwise assume good. For everything else (prototyping etc.), if in doubt toss it out and use a new one out of the bag or tape.
If you really must test it you can use a reasonable value series resistor (to limit current to maybe a mA or two) and a bench power supply capable of more than the rated clamping voltage to get a rough idea if it is okay and clamping. You can test it in both polarities. Just bring the voltage up slowly and observe what happens, comparing a multimeter across the TVS and the bench supply voltage meter. Out of circuit, of course.