I would monitor his computer usage for a while. My wife found that I was surfing Ashley Madison, which I was but I was not a member so I could not contact any of the women. To be honest, I was just curious to see what is out there in the dating scene, not to hook up with anyone. It was interesting and I'll admit, a turn on to check out the ladies, but again, I was just looking. If he is having actual communications with other women though, its a big red flag.