I am a polygraph. The police make people hold my hands and look me in the eyes so I can detect their stress. That's how I've met ~48% of my dates. I know bullshit when I see it. I can smell it. Think about that for a moment.
Everything you just read is bullshit.
Well, mostly. I really can tell pretty quickly whether you're lying. I feel it in my cockles. It's an inexplicable thing - a gut feeling, really.
Sometimes it manifests itself in your voice. A stutter, a hesitation, and most obviously - the pubescent crack. Granted, I'm fairly honest, and my voice cracks on some words. That has to do with how much air passes through your throat when you speak. Often, your voice cracks if you speak after swallowing heavily. But hey, people often swallow before lying; it bides time to think. That's another form of hesitation.
It also could appear in the strength of your voice. Many people are afraid of being caught in a lie, so their heart rate increases. As the heart pumps faster, more oxygen is needed, and the liar sounds out of breath. At the beginning, end, (or both), of the sentence, the liar should theoretically take a deep breath. At the very least, they are visibly breathing heavier.
Pupils dilate, skin pales, and sweat may form. Blushing usually indicates being embarrassed of the truth. If the suspect looks right or up before answering, they are thinking of the truth. If they look left and in either vertical direction, they are getting creative. Sometimes, not deflecting eye contact at all is a sign. The more open their body appears (uncrossed arms, open legs), the more likely they are being honest. On the other hand, the more closed they seem (pursed lips, dilated pupils, furrowed eyebrows), the more likely they are full of shit*. It's very Freudian.
It's amazing how much people actually lie about things that don't matter.
_
*They could also just not like the truth. Use discretion and take all of the other factors into account.