1. They place their lies in true places, stories, persons to have a base to drawn on. The best way to determine what exactly is a lie or a fantasy is to scrutinise every single detail of the story told.
2. Professional liars can force themselves to believe in their lies. That makes them more exposed to their own fantasies, and their lying can be ported to a subconscious level. In turn, it leads them to lie unconsciously.
Small unconsciously told lies are much easier to determine. That's why you'd need some more time, and more contact with the person suspected to be a professional liar.
Also, professional liars:
3. Avoid using big words, which are usually used by regular liars when lying, or avoid any unnatural way of speaking.
4. Mingle lies with casual conversations or lie in advance (before somebody finds out e.g. something was stolen or his delicious sandwich eaten).
5. Look you in the eye, and avoid to stare.
6. Play dumb or very stupid.
7. Often fantasy. But you need to determine how the person behaves normally: a high-ranking official will be capable of a financial fraud to get more cash, but rather won't jump on a bungee to steal a passerby's backpack.
Some useful questions:
What is his occupation?
What is his social status?
What friends does he have? (+ What friends DID he have, and why he doesn't have them anymore?)
In what living conditions does he live?
What hobbies does he have?
I had a school friend who was very capable of some stupid things (I've seen many myself). When he would like to tell them to a complete stranger, he wouldn't believe him, since he doesn't know that he's quite mad. ;-D That's why it is ALWAYS necessary to dermine who/what is the person before you even start suspecting him of lies or fantasies.
8. Admit to some very small lies to distract from bigger ones.
9. Remember when they lied, what they lied about, and the lies themselves. The problem is they slip their lies into their subconsciousness, thus leading themselves to lie more easily. And the more they lie, the easier they make themselves to be exposed as liars.
10. Fake a memory loss or that they don't remember something.
11. Are aggressive instead of being defensive, as regular liars do.
12. Avoid to change the subject of the talk.
13. Avoid to add extra details.
14. Anticipate questions or gather knowledge required to answer some vital questions, e.g. you tell your friends you're going to the movies, but you have to help your parents. Therefore, you need to know what movie was all about, and also need to choose your favourite scene.
15. Professional liars can lie about something you can't really prove or look up, e.g. a boy lying that his stomach hurts from eating beans - even if he would go to a school nurse, she wouldn't be able to determine what he had eaten, wouldn't open his stomach or prove the lie one way or another.
Also, pro liars deviate AFTER the lie / the talk with you and they often are content with their successful lies. Watch them closely when they walk off. Are they relieved you ended a small conversation? Overly enthusiastic or happy? (I.e. they were gravely serious when you met them, and your conversation contained very little good humour, it could be an indication that something is wrong.)
The major difference between a regular liar and a professional liar is that the latter can cheat you with the body language, and they are much much more smarter when lying.
Regular liars lie with words (and they can be spotted by establishing a baseline and seeing how they deviate from their normal behaviour)
Professional liars lie with both words & body language
Also, you need to remember that human's nature seems to sound truthful, i.e. humans were made sincere. That's why they give their lies away with their body language, or rather their body wants to show that its owner lies (does something against his nature).
Therefore, there always will be some unnatural body signs indicating the person may be lying. In some cases, such assumptions can be proven wrong, so using logic or having more examples of person's behaviour and statements can be more insightful.