To know someone is lying  or not we can use lie detector. This is a simple lie detector that can be made in minutes. It works to detect the someone who telling lie but it is not as sophisticated as the ones the professionals use. Here is the schematic diagram of this circuit:

Simple Lie Detector jadi1 circuit schematic

The skin resistance will go down when someone telling lie and this circuit works by measuring it. We can use electrode pads, alligator clips, or just wires and tape as the electrodes. We have to adjust R2 to position the meter at the center at the beginning of the interrogation (relax condition), then we can know someone is lying when the meter changes in response to a question. [Source: aaroncake.net]