To know if your partner is lying can come down to instincts most of the time. You begin to really know a lot about your partner. You learn to understand if something is bothering them. You begin to have the ability to tell if they're happy, sad, or mad without any explanation. So you become quite aware of your partner. Although we can all be blind sided, even by the people we think we know very well. Everyone seems to have a public, private, and secretive side to them. Lying, whether that's a cheating spouse, or just lies in general, can cause an unhealthy relationship. Trust is the most important thing you need in relationships. Majority of relationship problems come from lying. It's hard to rebuild trust when someone lies. Infidelity seems to be the basic problem when it comes to lying. I think most people who are capable of lying, are probably capable of infidelity. Lying in relationships means cheating. For instance, especially in a marriage. Marriage means you're sharing a life with a person. You cheat them, when you lie to them. You cheat the relationship and specifically them. Lying means a person is more motivated by self interest, rather than that person and theirs. I realize everyone lies. White lies doesn't mean someone is out to cause destruction. Living in lies in relationships are what causes destruction. It's important to give trust in relationships, though. You shouldn't be looking to find your partner lying. You shouldn't give them doubt, when there's no reason to doubt them. It means you need to have open eyes and judge by actions. Sometimes a person has a feeling when their loved one is lying, but they don't want to see it. Use your instincts, but never assume. Don't accuse, if you really don't know if your partner is lying.
Their actions or statements make very little sense. Most people who lie enough, become quite good at it. However, a lot of times lies don't really make a lot of sense. Look for things that seem out of character. It's important to study your partner, where you have a familiarity with them and their behavior. You should be able to understand personality well enough. Be familiar with their routines, interests, body language, etc... If something just seems a bit off, or out of character, then this at least allows for suspicion. Remember to never go on assume just because it doesn't make sense. Don't accuse someone of lying, if you're not positive. Explain to them what they've said doesn't quite make sense. Ask for them to clarify politely. Maybe they'll explain any misunderstandings, or try to spin the lie a little further if it is one. When someone is lying, they'll tend to try to change the subject fast. If they are telling the truth, then they'll tend to be happy to clarify the misunderstanding.
Study how responsive they're being with you. If someone is cheating on you for example, they'll tend to act different. A person can act quite a few different ways. They can act more irritable around you, angry, or even act a bit too nice. Too nice, as in seeming to act as if they're guilty of something. Constantly apologizing for things, even when they have nothing to apologize for. More importantly, you'll notice an absence not just presently, but emotionally. You won't feel the same connection. You'll feel a lot of tension and anxiety with this person. Regardless of how they're behaving, tension is what to look for. Awkward silence is another thing to look for as well. Unless you've just started dated, awkward silence means something isn't right. Either this person feels emotional upset about something, or they're hiding something from you.
Test their reactions. If you suspect them of lying about something, bring up the subject you think they're lying about in subtle ways. Don't come out and ask them if they're lying. You can, but if they've denied it and you still don't believe them you'll have to do it in subtle ways. For example, you make comments on what a faithful wife you have. You compliment your relationships, saying "to bad more people don't have what we got." Sometimes maybe you just give them compliments, and see how responsive they are to them. Or how you value the honesty in the relationship. Look to see how they respond. Do they look down, and avoid eye contact? Do they have that guilty look on their face? Is there awkward silence, or act as if they want to change the subject? Or do they look you right in the eye and smile? Reactions are very hard to hide. Especially when you catch someone off guard
Inconsistencies in their stories. You'll always be able to find inconsistencies in stories. When you lie, it's hard to remember which lie you've told. A person tells enough lies, eventually they make mistakes. Anytime you think they're lying, write it down. Write down specifically what they said, when, etc. Make actual notes, so you yourself aren't forgetful. A few days, or week and see if they remember. See if their story has changed. Bring things up in subtle ways. Usually by asking a question, acting forgetful of what they told you is the best method. Don't wait too long, or they might honestly just forget. A few days, to a week people should remember. If they lied, they probably didn't though.
Lack of communication. A lot of times lack of communication is the easiest way to figure something isn't right in relationships. One who lacks in communication, can tend to not exactly be telling you everything that is going on with them. Now, some people just aren't very good at communicating at all. It doesn't mean they're lying, they just don't really care to express themselves to others. However, if you're partner in the past has been effective when communicating, no long is, then something isn't right. They're not telling you something. It seems they're neglecting you and hiding things from you. They are either lying to about how they feel, or what they're really doing. People don't just lie with actions. Meaning with words, or other partners. People lie emotionally too. They don't always share what they're really feeling. Actions might be more malicious, but emotional lies can be just as troubling. Both acts can help ruin relationships.
Do your investigation. If you think without a doubt they're lying, investigate. Check and see where they really go. Check phone records and credit card payments if you really believe they're having an affair. Their actions and lack of communication is telling you that. Put the test to them, and try to catch them in the lie. Meaning gain the proof ahead of time, then ask them a question related to it. If it's more difficult to figure out, consider a private investigator. Those who lie, tend to lack communication. They tend to not make a real effort to explain themselves. Or when they do, it still leaves a lot of confusion, or things that lack sense. Obviously you don't want to make a habit of snooping on your spouse. People do lie, so if you feel your spouse is, then it's understandable on trying to get to the bottom of the truth. You want to give your spouse the benefit of the doubt first, though. Meaning allowing them to explain any misunderstandings. Also learn to take them at their word, unless it's proven to be blatant lie. Lies tend to catch up and being exposed. No lie stays covered. Sometimes the best way to figure out a lie, is by just letting the truth expose itself.