How to track the exact location of my wife's iPhone?

I need to know how to track where my wife goes and not let her know about it. I've seen it on the maps app that comes with the phone on a coworker that used to work at my job. I have an iPhone 5 and so does she.
I don't want to have to pay for this service i just want to know where she is.

You can't get that kind of service without paying for it. That would require some kind of tracking software to be installed on the device, and if she finds it, she'll know you're spying on her.

Besides, if you feel this is necessary to do, you have a far bigger problem. I suggest marriage counseling.