Unless you have a friend who owns and is trained to use a polygraph machine, it is very difficult to find a free lie detector test. In addition, many experts question the validity of polygraph machines.
By themselves, polygraph machines don't state whether the subject is lying or telling the truth; it's up to the person giving the test to make a judgment based on feedback the machine provides. As a result, people looking to take a polygraph test need to rely on someone trained to administer it, and convincing a professional to give a test for free can be difficult. In addition, the fact that a polygraph test needs to be interpreted has led to questions about the validity of test results, as different people giving the test might give different interpretations of the results.