1)Look for micro-expressions. It's something like a quick second of emotion that flashes across a person's face for a brief moment. 2)Notice the person's eye movements. You can usually tell if a person is remembering something or making something up based on eye movements. When people remember details, their eyes move to the left if they are right-handed. When right-handed people make something up, their eyes move to the right. Same goes for left-handed people. (strange but true :D) 3)Do not use eye contact or lack of it as a sole indicator of truthfulness. They are too nervous sometimes while telling the lie, so they would try to avoid as much contract as possible. 4)Check breathing. A liar tends to breathe faster, displaying a series of short breaths followed by one deep breath. AND.. that's all I can think of right now..xD hope I helped :)
Hard to. Gut instinct is the way most do but it is unreliable. Eye contact? Some people avoid eye contact out of shyness, some Asians out of respect. Only way is really lie detector. Even that is faulty.