Women cheat!

The only way to tell if a woman (wife or girlfriend) is really cheating is to talk to her. Tell her how you're feeling and that you have some concerns about the relationship. If she gets uncomfortable or confused on what to answer, you may have room to worry. (If asking her doesn't work, the only other thing I can think of is catching her in the act and this can be hard. Although, I think there's a tv show called Cheaters that has the resources to actually follow a partner around and tape any indiscretions. But I think you'd have to be willing to tape all of it and air it on tv if you want their help.)

No easy way to detect it. If a women is smart enough you won't be able to notice her cheating. I suggest if you suspect cheating simply ask your wife. The bottom line is that if you suspect such a thing the situation must be bad enough as it is so you better talk about it and work things out.

you cant tell you just take it how it is I guess that what I do,,, like you said women lie and are coniving they can kiss you smile and expect you to think everything is alright. I suggest talking to her directly and watch her reaction

You hire private detectives to follow her everywhere she goes. You check all her communication devices on a daily basis, you place bugs and cameras in her car and house. You make sure that she doesn't breathe without you knowing it.

or you meet someone, you are honest with her from the very beginning, you don't lie, you don't cheat, you don't put anything in front of her, you make love to her as well as you have wild crazy sex with her, you  talk to her and you listen to her, you give your life to her and the eventual family and you love every minute of it

and you read this book "BRAIN SEX" 

Good luck 

Treat her right and with respect! And you have nothing to worry about! When a woman lies and is caniving it's because in her life men have damaged her doing the same! Women start of giving there all! It's up to the men to appreciate, and respect it and not take her kindness for weakness! Which is most mens problem! Good luck!