Have you ever been talking to someone and started to wonder if they were lying or telling the truth? While it’s not possible to become a human lie detector, it is possible to make yourself aware of the little signs or “tells” that indicate the possibility someone is lying. Most often this is done by observing their body language rather than what they are saying.

The first tell you can check for is whether their posture seems stiff or loose. When you ask them a question and they answer, do they become stiff all of a sudden, especially in their upper body? This could be a sign that they are not telling the truth or are uncomfortable about the question for some other reason.

The second thing to watch for is the nature of their smile. If you ask someone to go to the movies with you, and they smile but something in your gut tells you it’s not genuine—quickly glance up at their eyes. With a genuine smile, their whole face should light up and they will get little crow’s feet around their eyes. If instead, it seems like only their mouth is smiling, odds are it’s a fake smile or they aren’t fully feeling it.

Next, if you want to check for truth-telling, notice what someone does with their eye contact. A person who is not telling the truth may do one of two things: they might avoid eye contact altogether because it makes them uncomfortable, or they might stare you down in an aggressive way to try and make you believe them. On the other hand, someone who is telling the truth will probably balance a normal amount of eye contact with looking away.

Finally, if you’re still not sure if someone is telling the truth or lying, try listening to what your gut feeling is telling you. Often, even if someone is skilled at being deceptive, their face will have micro-expressions that occur too quickly for your conscious self to observe. However, your subconscious does pick up on these and may reveal them to you through your gut feeling. While it’s never good to jump to conclusions or accuse someone without proper evidence, it’s also important not to ignore your gut feeling. Quite often it can be right.

All that is to say that you should never assume someone who shows nervous body language is necessarily lying. Some people, especially those with social anxiety, may just come across as nervous by default. It’s not that they are trying to hide something from you, other than perhaps their anxiety. So no matter how good you become at decoding body language, it’s always important to take the context and the person’s unique characteristics into account.

So to recap: if you want to judge whether someone might be lying to you, observe their posture, their smile, their eye contact, and also your gut feeling. Combining data from all these sources will help you to make an informed decision and take it from there.