There are a few ways to tell if someone is lying. Firstly, if someone struggles to keep eye contact with you and are looking to the side a lot then it could mean dishonestly. Similarly, if they are playing with their ears or hair then that’s a sign of discomfort relating to what they are saying. The subject matter should give you further indication as to the truthfulness.
For example, if they are talking about something that makes you uncomfortable then the signs above probably don’t show they are lying. However, if they are recounting a story or explaining themselves to you and do the above then take their words with a pinch of salt.