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 .
A very nice and simple AC/DC indicator circuit diagram is presented in this page . The AC/DC indicator circuit diagram is based on two transistors one PNP and one NPN transistor . The circuit will give a visual indication using two LEDs . Also this AC/DC indicator circuit will provide an information if the signal tested is positive or negative .
If the red LED will glow the signal is positive DC , if the yellow LED will glow the tested signal is negative .
When the tested signal is AC , both LEDs will glow ( LED1 and LED2 ) .