It’s fortunate that liars’ pants don’t really catch on fire. If they did, much of the world’s population would be ablaze at any given moment. “People lie most days,” says Kevin Colwell, a professor of psychology at Southern Connecticut State University who researches deception. The majority of fibs are the sorts of harmless white lies that don’t hurt anyone. (You don’t actually like Aunt Mildred’s new haircut? She’s not going to be any worse off believing otherwise.) “Lying is a very important social skill,” he says. “It greases the wheels of society and makes our relationships work better.”

More harmful lies, however—those that are intentional misrepresentations of material facts—have the opposite effect. Being lied to or about can deteriorate someone’s mental health and lead to a ripple effect of outcomes in their life, Colwell notes, including destroying their relationships. That’s why many of us play detective, trying to separate fact from fib.

So how can you tell if someone is lying to you? The key is to enter the conversation without assumptions, says Colwell, who has trained law enforcement officers and investigators with groups like the National Counterterrorism Center in deception detection. “You have to not go into it assuming they’re lying to you, because if you do, it’s going to change how they interact with you,” he says. “You have to be engaging and connect with them in a way that makes the conversation easy for them.”

You won’t always be able to tell for sure. But here’s what to look and listen for if you’re trying to figure out if someone is lying to you.

There will likely be physical signs

To know whether someone’s lying, you must have a baseline understanding of what’s normal for that person, explains Jim Clemente, a former New York State prosecutor and retired FBI special agent. The more time you spend with them, the better you’ll be able to recognize changes in behavior and communication. Some people, for example, might struggle to make eye contact in every conversation; for others, it will be a tell that something is amiss. Once you know what their norm is, pay attention to physiological changes caused by the fight-or-flight response: someone who is lying might start sweating, or their pulse will visibly pound in their neck. As their heart rate and blood pressure shoot up, their salivation will decrease, which means you might notice they’re gulping or licking their lips. Some people will begin fidgeting. “Shifting their torso, moving the chair away from the person talking to them, standing or attempting to leave—I’ve seen all of that,” Clemente says. “You might see rubbing or wringing of the hands, pinching parts of the body, clearing the throat, pulling on the hair. All these things can be stress and tension releases.”

They’ll repeat the same story over and over

Let’s say you ask your partner what they were doing on Wednesday night. After they fill you in, respond: “That sounds neat. I wish I had been there—tell me more about what happened.” People who are telling the truth tend to talk in a natural, free-flowing way, Colwell says—they aren’t worried about getting caught. So they’ll supply new, relevant details they didn’t include the first time around.

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People who are lying, on the other hand, tread carefully. They might talk a lot, but “they’ll tell you the same thing they just told you, the same as how they already said it,” Colwell says. “They’re making sure they don’t contradict themselves and give information that could lead to them getting caught.” If you’re hearing a whole lot of the same thing, continue asking specific questions—and it will likely soon become obvious that you’ve discovered a lie.

They’ll be oddly chronological

People who are lying tend to tell stories chronologically, Colwell notes—as opposed to those who are being truthful, who will go from the most important parts to the least important. If you don’t have anything to hide, “the first thing you’re going to remember is the most important piece of that event, and then the rest of it will come back,” he says. “If you know you’re going to lie, you’ve practiced and you have a script—and scripts start at the beginning and end at the end. Scripts don’t start in the middle.”

They’ll speak more eloquently

Surprisingly, people often speak better when they’re lying than they do when they’re telling the truth. “They’re engaging in impression management,” Colwell says. That might mean using a more complex and sophisticated vocabulary than you would expect, with words you didn’t even realize they knew. So if you’re marveling at your friend’s newfound language mastery? “It’s a clue that they’re lying to you at that moment,” he says.

They’ll drop or change pronouns

When a woman asks her lying husband about his day, he might reply: “I went to the park. Ate lunch.” Notice that he dropped the “I” in front of “ate lunch,” Clemente says. That could be because he’s covering up that he and another person—“we”—ate lunch. Similarly, he recalls suspects who said: “Left my house. It was on fire.” His response: “You left your house because you discovered it was on fire, or because you lit it on fire?”

Their sentences may be full of qualifiers

Some lies are indirect: People omit crucial facts or feign forgetfulness. In these cases, they’ll often answer questions with questions, Clemente says. Consider the famous scene in Seinfeld where Elaine’s friend asks her if she’s having an affair with George. “Why would you think I was having an affair with George?!” a frazzled Elaine responds. (Spoiler alert: She wasn’t, but she was covering for George, who had gone on a date with Marisa Tomei.) Or, when asked if he killed his brother, a murderer might respond: “Why would I want to hurt Jack?” If you ask a yes-or-no question, Clemente says, pay close attention if you don’t get a straight response.

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People who are lying by omission also tend to be vague or evasive by using phrases like “I think,” “probably,” “sort of,” “maybe I was,” and “I started to,” Clemente points out. “If somebody says, ‘I started to drive to work, and then when I got there….,’ they probably edited out what happened in between,” he says.

They’ll sound different

Paying attention to non-linguistic verbal cues—like tone, volume, pace, and pitch—can be revealing. People who are telling the truth usually speak in a consistent way, Clemente says; those who are lying are likely to have a broken rate, with variations in pitch and amplitude. “If somebody’s under stress, their pitch might go up quite a bit,” he says. “Their pace might slow down because they’re trying to think, or it might be really fast because they’re so nervous.”

Their eyes might hold secrets

There’s a common assumption that people look away when they’re lying—but actually, research suggests that gaze aversion is common when we’re thinking, which doesn’t necessarily equate to telling a fib. “People who have a lie ready to go often look at you straight in the eye, because they want to know if you’re going to buy it,” says Wendy Patrick, a longtime prosecutor and author of Red Flags: How to Spot Frenemies, Underminers, and Ruthless People. “They’re trying to see if you’re buying what they’re selling.”

Plus, people who are lying usually don’t smile with their eyes, she adds. Generally, if someone is telling the truth, “you’ll see their crow’s feet as their eyes relax into their smiles.” It’s probably a safe bet to trust what they’re saying.

Their mannerisms won’t match the situation

When people are truthful, their mood and mannerisms should match the message, Patrick says. If you detect a visual-verbal mismatch, consider it a red flag. That might mean smiling or giggling while discussing a serious subject—like Christopher Watts famously did in 2018 when police interviewed him about whether he murdered his missing wife. (Though Watts initially maintained his innocence, he later confessed to killing his pregnant wife and their two daughters.) “It doesn’t always have to be something so sensational,” she adds. “But it is the dynamic of how our emotions belie our words, and indicate that we’re lying.”