There are many different things people call someone who lies all the time. A person who lies all the time is often called a liar or a habitual liar. They can also be called dishonest or untrustworthy.
Deceitful and deceptive are two more words that people can call those who lie habitually. Fraudulent and misleading are also used to describe someone who lies all the time.
When someone lies all the time, it can be hard to determine when they are telling the truth. Some people study a person's body language, breathing or eye movements to determine whether or not they're telling the truth, although it's not always possible to detect a lie using these methods.