It’s very easy to test the polarity of a signal at a point from circuit ( if the voltage from test point is positive or negative ) by using a digital multimeter or using an null detector electronic circuit like in this diagram . This null detector polarity tester circuit diagram is based on a common 741 operational amplifier that require few additional components to do the null detection . The test result of the polarity indicator is displayed through the two LED’s LED1 and LED2 .
This detector circuit diagram need to be powered by a 12-15 volts DC power source .
For a easy identification of the null you can use two different color LED’s .