Your gut instinct is telling you that something isn't right. Maybe your partner has started to act suspiciously or secretively, or they have become suddenly distant.
The first step in catching a cheater is understanding the signs of cheating. If you notice your partner is acting differently, it could signify that they are cheating.
They're always secretive about their cell phone, phone calls (especially incoming calls), text messages, and computer usage (whereas they weren't so vigilant before)