There are many signs your wife is cheating. So many, in fact, that you might be sing someone without even realizing what they are. You don't need to know every single one but there are a few that are a little more revealing than others.
Just remember that these signs could be benign or could be signs of a cheating wife. Don't assume anything without proof but be more diligent in your search for proof if you see two or more of these signs.
1) Your wife has suddenly started spending a lot more time with "the girls." Ladies night once a month is one thing. When your wife starts having two or three nights with the girls in a week then there might be a problem. View everything in light of how it impacts your marriage. Do you feel that your wife is spending too much time away from home and not enough time with you? Whether it's another man or it really is lady's night, if it's impacting your marriage in a negative way it's time to work on a few ground rules.
2) Your wife has suddenly become more critical. You're probably sitting there thinking "yeah right, like that could happen," right? And yet it does happen, often, when women cheat. She wants to find a way to justify her cheating and often does so by finding fault with her husband. If she suddenly starts becoming more critical of you and it isn't related to hormones then another distinct possibility is another man.
3) She's never home anymore and when she is home, she's so distant she may as well be somewhere else. The distance that is created by affairs is more than just a physical distance. Especially among women having affairs. It's more emotional for the average woman. This means that she is going to pull away from you in order to give more to the other guy. Most women are capable of giving their all to two men at once. As the relationship with the other man deepens you'll begin to notice a steadily growing distance between the two of you.
4) Your wife has suddenly stopped confiding in you. In fact, she's pretty much stopped talking to you altogether. There are many reasons for this when it comes to cheating. She's afraid of saying too much. She doesn't want to hurt you. She's holding on to no small amount of resentment towards you. It could be any of these or something else completely. The bottom line is that she's not talking to you anymore and that's a huge red flag in most men's books.