Find an ohmmeter resistance between two contacts of approximately 300 ohms by checking each tip with each. Then connect the 12V power supply to this resistance. You can check with an ohmmeter alone if there is a transition between the other two contacts but this check does not give us information about the state of wear of the relay. To check its condition, it would be necessary to connect a 60W car bulb in series with a 12V battery or 12V / 5A power supply in series with the remaining free contacts, and watch this bulb for one hour by measuring the voltage between the relay contacts that are connected to this bulb. The voltage at these contacts should not exceed 0.1V during this hour of operation. After disconnecting the voltage from the terminals where there is a resistance of 300 ohms, the bulb should go out. You can connect a larger number of light bulbs under the relay, because if the relay is at 40A, it may not connect correctly at a current of e.g. 20A when its contacts heat up.
