Lie detection is Maggie and Charles's ability to sense if someone is lying to them.
Maggie can sense if a person is lying within her presence.
Maggie can sense a lie by being in close proximity to someone. A person didn't have to speak for her to know if he or she was behaving in a way contrary to their beliefs. If anyone misrepresented themselves in either appearance or action, Maggie could feel it automatically.
Maggie can only sense when someone is lying, without necessarily sensing the reason for it or the intentions of the liar.
This power also has a side-effect, as any kind of deception makes her physically uncomfortable.
When Maggie was human, she always knew when a person was lying with their words, which compelled her to always point out the lie. This made the inhabitants in the village feel uncomfortable around her as they could never lie in her presence, prompting her parents to leave her behind to care for her grandparents when they immigrated to America - she was well aware that her parents did not love her as much as they did her other siblings.
When she crossed paths with Liam and Siobhan, Siobhan offered to make her strong and never have to starve ever again; she sensed her honesty and agreed.
By becoming a vampire, she proved a good asset to them; no lie as ever escaped her detection. Her lie detection was intensified, allowing her to read peoples' lies not just by their words, but also by movements and facial expressions, though any deception made her physically uncomfortable as a side-effect to the change.
Maggie found herself well-fitted in the coven because Liam and Siobhan were always truthful to her and to each other.
When Maggie and her coven is approached by the Cullens' cry for help, Maggie senses their honesty and agrees to be their witness against the Volturi. Even after Alistair - one of the witnesses - fled for fear of the Volturi, Maggie remains determined to stay behind and help protect the truth.
During the Volturi confrontation, when Aro tells Renesmee he has no desire to hurt her family, Maggie hisses at his lie.
Charles can sense when a person is lying to him. It has not been explained how different from Maggie's this gift is, but it appears to be less pronounced than hers.
Charles developed the power to detect lies after his transformation to vampire. In late 2006, Charles and his mate, Makenna, are approached by Renata, who asks for their witnessing to the Volturi's cause as they lay punishment on the Cullens; he senses no dishonesty from their behalf and so agrees to their wishes.
When they arrive in Forks with the Volturi to question the Cullens' doing, Charles senses the Cullens' honesty in their testimony, being falsely blamed for a crime they did not commit, and also newfound dishonesty from the Volturi. He gives his witness and then leaves with his mate, hoping to avoid a chaotic outcome.