1. Notice their gestures. Lying is uncomfortable for most people (except sociopaths). So, in general, people will show some kind of evasion in their body language as they speak the lie. It could be a step back, a turn to the side, placing a object in front of them, cross an arm or leg, shifting a hand or foot, or just a self scratch.

2. Most people will try to avoid a complete lie. They will prefer to tell most of the truth but not include the most relevant or crucial detail. Or they will deflect by changing the topic or answering in an obscure or vague manner.

3. Incongruence - the emotional tone seems inappropriate - doesn't match the situation. People can have a wide range of emotional responses, but usually the responses will be typical of that person's personality - how they would respond in that type of situation. If they don't, something's off and it's probably some kind of lie.

4. They are an addict. Lying has become a normal way of hiding their habit and its consequences.

5. In the end, one has to do the work and find the facts. That's the only real proof.

Keep asking the same question million times like done here.

Depends on the lie but going full on pretending to believe the lie often works because in most cases the lie when scrutinized doesn't add up.