how will you know that he/she is lying?

Okay, there's no uniform signs because everyone's different. If you know a person well, you may be able to read their "tones" in context to what they're saying. If they're a stranger, you may wish to look for nervous behavior or "their tell". A tell is like a nervous tick that a person does subconsciously. Even if they're otherwise very smooth or cool, they may still be using their tell. The most common is playing with one's hair and blinking, but everyone's is different and the more experienced liars usually know about tells...and at that point it's about catching indescrepencies and flaws in their logic when they're talking to you. Plus a little intuition never hurt much. It's an art and a science. You might want to try to learn some body language?