Lie detector tests are well established and well understood methods for assessing whether a person is telling the truth or lying in response to specific questions. Provided that the person subject to a lie detector test is fully aware of the purpose and potential consequences of taking a lie detector test and provided that such tests are carried out in an appropriate environment in accordance with proper protocols and safeguards then there is no reason why lie detector tests cannot be validly used for law enforcement and prosecution.
Lie detectors do not exist. There are polygraphs and various machines that measure indicators that may point to lies, but they also measure stress. People can pass them with training and they pick up a lot of false positives. In some cases they have been shown to be about 50% accurate.
Lie detector tests should not be used by law enforcement or the judicial system. They are not perfect because everyone is unique. Different people will have different reactions in certain situations, so deciding that someone is guilty or not guilty based on a polygraph test is very unfair and will lead to wrongful prosecutions.
When you're stressing out, you can give signs you are lying. It is impossible to be neutral and this particular case. Experts can simply see evidence of a person's lying. So for me, there is no reliable way to determine what's wrong or right during a lie detection procedure. As, a poker player, we can be blurred by false response and non verbal behavior.
Lie detector tests are not a valid tool for law enforcement and prosecutors, nor should these tests be admirable in court. These tests are notoriously inaccurate. A suspect might already feel anxious about being under investigation skewing the accuracy of the results. Also, someone might be a really calm and cool liar; in which case the lie detector would not detect dishonesty. In short, lie detector results are not the same as hard evidence.