For me it was a combination of a gut feeling that something was going on, less sharing/talking of what was going on at his work, he no longer talked about the OW at work, he went into bathroom or garage to use cell phone, checked his voice messages constantly, and he said they were "just friends".
When I asked him to just call OW in front of me - he said he felt smothered and needed privacy. He withdrew more.
It took awhile and huge self control on my part to not ask about OW - stop sacrastic remarks when he went to bathroom with cell phone - etc. But I think that went a long way to help the stich. That and me GAL.
