Usually when someone lies, they feel a little uncomfortable and worried about getting caught. If your BF is lying to you about something big, chances are his attitude will change a little bit. If he's usually really open with you and very happy and he's suddenly being secretive and jumpy, that may be a sign that something is up. Ask him what's going on - if he denies that anything is wrong or gets annoyed with you quickly, it's totally possible that he's keeping something from you.
He Doesn't Look At You
Pay attention to your boyfriend's body language whenever he's telling you something that you think he's lying about, or whenever you ask him if he's lying. Will he look you in the eye? If he'll look anywhere except at you, that's a good sign that he's not being honest. It's hard to look in the eyes of someone you care about and boldly lie. If you think he's full of it, say, "look me in the eyes and say it." If he can't do it, he's probably lying.
His Story Is Never The Same
Lying can be difficult, especially if you're lying about something complicated. Pay attention to the details your boyfriend gives you when he's telling you something. Wait a few days, then ask for the same story again. If some things don't add up, it could be because they didn't happen (or maybe he just has a really bad memory - you know him better than I do!). If this happens more than a few times, it could mean he's making stuff up and isn't good at keeping track of everything.
He Gets Super Defensive
How does your BF react when you accuse him of lying? If he gets super, super defensive, it might mean he's lying and is freaking out about getting caught. This one is tricky, because it's completely natural to get defensive whenever you're accused of fibbing, whether you did it or not. However, if your BF's response is over-the-top, it could mean he's in panic mode.
There Are A Lot Of Long Pauses
According to Cosmopolitan, long pauses before an answer are a sign that he's not telling you the truth. See what happens when you say something like, "Where were you last night?" If he pauses for a minute before answering or repeats the question before responding, he could be giving himself time to come up with an answer. He Accuses You Of Lying All The Time
Sometimes when people feel guilty about their own lies, they try to shift the blame a little bit. If your guy is constantly accusing you of breaking his trust, it may be because he feels bad about breaking your trust. Or, if he counteracts your probing question with one of his own, he could just be trying to distract you.
He Never Gives You Any Details
Do you think your BF is lying about who he was hanging out with earlier? Try to ask some innocent-sounding questions like, "what did you do?" "did so-and-so say anything about what happened in class the other day?" Don't grill him for details, but ask a few questions. If he keeps saying, "Um, I don't know" or "I don't remember" or if he blows up about it, it's probably because he doesn't have any real details to give you.
He's Super Weird With His Phone
I would never advise you to take a secret peek at your boyfriend's phone, but keeping an eye on how he acts with it is okay if you suspect he's lying. Does he constantly keep it with him, even if he's just running to the bathroom? Do you notice him deleting every single text he gets? Does he freak out if you touch it when it goes off? If he's crazy private about his phone, it could be because there's stuff in there he doesn't want you to see.
He Has A History Of Lying
I don't like judging people based on their past, but sometimes you really can find out things about a person based on what they've done before. If your BF has a history of lying to you, what makes you think that this time is going to be any different? If he's lied to you once or twice about little things, that doesn't mean he should never be trusted. But if he's fibbed about some major stuff before, he could very easily be doing it again .
You Have A Gut Feeling
Honestly, in the end, you really need to go with your gut. Like I said before, you know your boyfriend better than I do. You can go through all of the steps above and still not find out for sure if he's a liar. But if you have a serious gut feeling that he's being dishonest, then you probably have that for a reason. Trust yourself - dig deep down to see if you really believe he's lying or not. Sometimes we have these feelings because we're right.