Observing and becoming aware of elements of body language like gestures seems to be fairly simple, but interpreting them is something else.

Reading body language needs knowledge and also individual preparation.
You need to know how a person normally behaves so that you can spot meaningful deviations.
This includes knowledge of the individual's culture.
Don't judge the body language through the bias of your own culture.

One of the most common errors a novice in body language can make is to interpret a solitary gesture in isolation of other gestures and the context.

Don't try to find meaning in a single gesture.
Gestures come in clusters and the sum of them reveal how a person feels.
These are the full range of expressions and movements that we see.
We weigh up the total of these and come to a conclusion about mood, character and thoughts.

It’s advisable to relate gesture clusters to the context in which they occur.
This means that the external environment also influences gesture clusters.

And some kind of body language (the so called minimal cues or micro expressions) is so inconspicuous and may only appear for the blink of an eye lid.
It needs a lot of training and awareness to identify such minimal cues.

And there is another aspect to be taken into account; if someone believes in his "lie" the chance is high that there are only a few or no cues at all.

Reading body language is not 100% accurate all the time.
But by combining all the cues, you'll come up with reliable data supporting your assertions.
There are a lot of aspects to be taken into account to make an educated guess as to whether someone is telling the truth or a lie.
Knowledge and training are essential to be able to accurately read body language.

With individual tuition and coaching you will learn to be more perceptive. You will read the gestures and accurately match them with the verbal statement.
This makes it easier for you to interpret more accurately.