The method includes appearance inspection, resistance test and winding test.

1. Appearance inspection: check the transformer for obvious damage or scorch marks, and pay attention to whether the iron core is loose or hot.

2. Resistance test: Measure the resistance of the transformer with a multimeter. The resistance of the main transformer should be around tens of ohms, and the resistance of the auxiliary transformer should be around hundreds of ohms. If the resistance value is abnormal, there may be problems such as poor contact of the coil.

3. Winding test: Test the winding of the transformer with special test tools, and the test results shall meet the specified values. If there is deviation, there may be something wrong with the transformer.