If you suspect that your husband may be cheating but don't have any proof as he is too smart to leave any obvious clues; consider the following subtle signs that your husband may be having an affair.
All of the sudden his opinions on open marriage, polyamory or polygamy have changed. He used to be vehemently opposed to any non-monogamous arrangement, but now he claims it can be beneficial etc. He raises hypothetical questions such as, "do you think it's possible to be in love with more than one person" or claims that being monogamous is not really natural and so on. Be aware of uncharacteristic open-mindedness when it comes to love and marriage.
He's acquired new interests and tastes and these become apparent suddenly. In other words, you weren't with him when something piqued his interest and had absolutely no idea he is heavy into foreign affairs, feminism, soccer, chess, ballroom dancing... you get the picture.
If he stops having sex with you more than likely he is getting it somewhere else. Don't fall for the "I'm tired" and similar excuses, as no healthy man is ever too tired for sex. If he used to want a lot of sex from you (2-4 times per week) and now never initiates lovemaking he may be cheating. The other side of that coin is that he may suddenly want to have as much sex as possible. Not only that, but he wants to try new positions, techniques etc. Unless you and him have talked about and made a decision to spice up your sex life, he is probably getting some ideas on the side. Another thing to watch for is improved technique; unless he's discussed it with you and gotten pointers from you or you've been doing some hot research together, chances are he's getting some instruction elsewhere.
Suddenly his co-workers or friends are uncomfortable in your presence. If you notice that people he works with or associates with on a regular basis treat you differently, it may mean that they know something more about your husband than you do. He could be carrying on an affair at work (the most convenient) or perhaps his friends have covered for him in the past. Take notice if people start treating you with concern or pity.
He suddenly becomes very moody, judgmental or depressed. He picks fights with you in order to stomp out of the house and be alone. He starts asking whether or not you're happy but can't really come up with anything specific that would account for his unhappiness. He starts talking about all the things he's wanted to do but never got around to doing because he was providing for his family and taking care of his responsibilities. If your husband becomes nostalgic for his youth and bachelor days, beware.
If he starts mentioning another woman (or man) that he has never mentioned before with some frequency, that person may be the new object of his desire. This is subconscious. If he has fallen in love with another person it might become difficult for him not to mention them.
If you suspect that your husband might be cheating don't confront him until you have proof, as this will only alert him to the fact that he needs to be more careful. What you want to do is pay attention, observe him and collect evidence for a period of two weeks or longer. You might consider hiring a private detective to get the facts. Again, have the facts in order before you confront him.