It is a very difficult and challenging job to find out that who slept with your wife or not. It is even more difficult to find out hidden and secret relationships of any person especially your wife. So far your question that how could you find out whether your friend slept with your wife is a difficult one yet I will try to justify it to some extent.
First of all try to find out the business of your wife and your friend as well. You will have to become a detective to perform this job. Casually tell your wife and friend as well at the same time that you are going out of the city. Make your false programs in front of both of them so that they are deceived and if they are in love with each other or they have sexual relations, they will definitely make a program to sleep together. Then go away in their presence and try to seek out reality in a secret way. If they have such sort of relations, it will be exposed before you. Secondly give maximum chances to your wife and friend in isolation and observe them minutely in a hidden manner. I am sure by adopting these steps you will find out whether your wife sleeps with your friend or not.
I am one who has done that in the past, the cheater and the dog. I can tell you this, if she has slept with him with your friend or wants to then check her mobile phone of to how many times she calls him or messages him. When you see him and her check out their body language and eye contact with each other. If you feel theirs a connection with them then trust your instincts. Don't confront her or him, think about what your going to achieve out of this. If you want to divorce her then plan a way to catch them out. Get a party going at home with a few drinks, ask a good friend who you really trust to observe both of them. Get him and her drunk, set up a hidden camera or a voice recorder in the room. Get your friend that you trust to put them in a room to look at pictures or something else then leave. In that room is a hidden camera or a voice recorder, your friend and your wife who are now drunk will not resist temptation and will follow thru this moment. When they walk out of the room, go back 30minutes later and find the evidence. Go back to the party and tell everyone how much of a dog he is and how bad your wife is. Putting them on the spot at a party with friends will kill all their feelings and emotions. Your cheating friend will go home and your wife will ask a friend to stay over until she comes to terms with the situation. I hope that will help you out.
Finding the answer wouldn't be the hard part. Are you prepared for the answer if it happens to be yes? If you even sincerely suspect such a thing, then I would
#1 Lose the "so-called" friend.
#2 Find a marriage therapist.
If this is something you're wondering about may have happened prior to your marriage, as in, they were an item before you and your wife were, than let it go and be happy. Some questions are better left unanswered. If the latter is the case then it definitely should be water under the bridge. Good Luck.
I suspect my wife slept with one of my friends many years ago, when I was in grad school, and very distracted by the difficult course work. She used to flirt with him openly when he would come by, and she used to tease and tell me how sexy he was all the time. But then the flirting and comments about him stopped, though she still likes to talk to him when he calls. When I asked her point blank if she did, she responded "It makes me sad when you ask me something like that," but she never denied it. I made some references to them being together other times and she laughed it off, but still has never said "No, I didn't," despite me making it clear it was an issue that bothered me. When I mentioned his name in a conversation once her eyes darted quickly to the side, like she didn't want to look me in the eye when his name was mentioned. On New Years Day he called me but ended up speaking longer to her. As she made small talk with him, I stayed in the room, acting like I was not paying attention to her. She started playing with her hair, but stopped abruptly, as if she remembered she should not be doing that. I sept with a girl years ago and my brother in law met her once and my name came up. He said she got real excited, and fidgety and couldn't stop fluffing and playing with her hair. I am sure my wife did it with him, but I have no proof. I guess it was turnabout; I popped his wife's cherry before they started dating, but she was my girlfriend at the time. It really depresses me. I hate thinking that he has continued to act like my friend when he had my wife, and I hate that I'm sure she cheated on me and they got away with it.