The body can’t lie. That’s why lie detector tests are fairly accurate. Once you know what to look for, you can detect lies for yourself!
People will cover their mouth when they are holding back. If they are directly asked a question and do this, the next thing out of their mouth could be false. If others are speaking and you see this, there’s something important on their mind that they don’t want to say.
Whenever you see someone cover their mouth, further questioning is in order to get to the truth. It happens quickly and can look very natural. But don’t be fooled! Keep asking until you get to the truth.