Max, body language and voice stress are very useful. They do require training in order to apply them effectively. There is a common misconception that a certain body movement always means a particular thing, this is not true. Context and the individual's baseline habits are important to interpret them correctly. To be effective with voice stress, it really requires technology to detect the subtle aspects.

With training, language, body language and facial expressions are very reliable. However, a subject's baseline should still be established before it is highly effective.

With all of these tools, the amount of time you have to observe the subject is important. The longer you can observe, the more accurate your assessment will be.