As long as we understand the structure of the triode, it will be easy. The triode has two PN junctions, which can appear to have two diodes in series, and can be divided into NPN and PNP triodes. There is a PN junction between the BCs of the NPN tube, which has a single conductivity. That is, the black probe of the multimeter is connected to the B pole, and the red probe is connected to the C pole. The PN junction is conductive, and vice versa. Similarly, there is also a PN junction between BEs, which also has a single conductivity. In this way, the single conductivity of the PN junction can be used to determine whether it is an NPN or a PNP triode. As shown below

If a pin is found on a triode, connect the black probe to this pin, and connect the red probe to the other two pins in turn, which can be turned on, and vice versa, which can not be turned on, indicating that this tube is an NPN triode.

If a pin is found on a triode tube, connect the red probe to this pin, and connect the black meter to the other two pins in turn. If the probe is not connected, it means that this tube is a PNP triode.