If you've been around a person for a while, you learn their normal behavior. When someone is lying, they normally can't make eye contact, they will use their hands to talk, they tend to look away and change the subject.
If the person is lying about another person....they will use pronouns instead of using a name. For instance, if the person's name is John....they will use he instead of "John".
Also you will notice some slight changes in their facial expressions. It is well known that a person who is lying will slightly grimace at the end of talking...this can barely be noticed...usually only a trained eye can spot it.
There are ways to find out truths. Such as... ask a bunch of questions about the subject and be sure to remember the answers correctly---write them down if you must. But, don't let the person know you are questioning them to trap them. Ask the questions casually and spread out if you must. Then later, ask the same questions...not all together, but one by one over time...if a person is lying, usually they will make up something else. A person fibbing, eventually lose track of all the half-truths and straight out lies they've told---and they end up forgetting if they said they were at Sues or Mary's house. It's easy to catch them up!!!
Eventually lies catch up to people. Usually people will hang themselves over time. One lie, leads to another lie, and before they know it they have told one right after another to cover up one lie. Once a person falls in to this never-ending lie trap, they are destined to be found out.