In psychology, how to know if someone is lying ?

Can and can't : What makes poker fun is - you really can't tell reliably when someone is lying. If the person lying has a negative emotional response because they are lying, they may have sympathetic nervous system signs - heart rate, flushing skin, averted eye contact, word finding problems ; more confusing (non-direct) language, also may be avoidant or confrontative. Knowing the person well helps detect changes.

There is some: research on this topic. There are some popular books on this topic as well. David Lieberman, PHD wrote a book "Never Be Lied To Again." Basically, there are verbal and physical cues.

Depends: Under what circumstances? There are some tests that can indicate truthfulness.

Many things: Body language, facial expression, and other factors can give clues to whether or not a person is lying. A poly graph also can indicate whether or not a person is lying. Polygraphs measure galvanic skin response (in other words sweating,)heart rate, breathing rate, muscle activity, and other physical responses to questions to determine whether or not the subject is lying.