You can certainly ask him, but he can (err) lie some more. Don't be paranoid. Do your research first and then react accordingly. Not sure how? Read on for some signs that he may be lying. After you assess the situation, it's okay to take the next step.
1. The story always changes.
He goes from tale to tale and details don't seem to stay the same. He first told you he was out with the boys. Then, he said he was at happy hour with coworkers. If the inconsistency persists, he could be fibbing. The next step? We hate to suggest this, ladies – you may not have a choice – but do a little snooping. If he says he was with the boys, try to get the scoop from the boys.
2. He won't look you in the eye.
Experts say if someone can't look you straight in the eye, he or she is probably not telling you the whole story. No wonder your momma used this trick when you were little and she thought you were telling a tall tale.
Study his body language when he's justifying something or telling you something that you think he may be lying about. If he's not able to look you in the eye, the next step would be to just come out and ask him. By this time, you should be able to tell if he's truthful or not by using the eye tactic. (Watch a fun play on reading a fibber's body language on the TV drama, Lie To Me -- a pic from which is below.)
3. The details are off.
Sometimes the details will be really obscure, or perhaps there are too many details. If your man gets too specific and is giving too much information, he may be compensating for something.
People who are lying tend to over tell because they're hiding the very thing that they're revealing. If the man in question is giving you random details about an event or story, pry a little bit – ask some questions about said details. See if he trips on the specifics.
4. He's done it before.
Okay, it doesn't take a brain surgeon to tell you that if he has lied before, he may lie again. Not to throw the man under the bus or anything – of course, it would also depend on what he's lied about in the past – but you can never trust someone who's capable of making up stories too easily.
Now for the next step... If he's made lying a habit and the lies are too huge to ignore and forget, you may want to consider leaving the relationship. If he lies here and there, and they're minor fibs, talk to him and get to the bottom of what's with all the lies. It might be the result of an insecurity. And if that's the case, then reassure him that he can be honest with you.
5. You have a gut feeling.
If you just know, we've got bigger problems. Whether he lied or not is no longer the issue if this is the case. You just don't trust the man. And what's a relationship if you don't have trust? Sit with your thoughts and come to grips with why you don't believe him. If the relationship is that important to you, after you do some soul searching and thinking on your own, sit him down for a talk. Work things through – find out what the problem is, and try to remedy the situation before it gets even more difficult.