Your gut tells you once you have been intimately involved with someone - you really don't have to put your finger on it, you just know but they've denied it and you don't have physical proof.
So how to catch a cheater? Physical evidence could be a compromising photo, dna from a sexual encounter or a confrontation involving a third party. OR, you actually catch them doing the dirty at some level of nakedness and surprise the catshit out of them. Having a phone handy is a good idea to collect the evidence. But HOW do you actually get that chance? It can be actually pretty easy to set up by a regularly scheduled out of town visit that your significant other will take advantage of either by inviting their secret lover over or by meeting them at a hotel, motel or their place.
How to catch them cheating is again, pretty straight forward. Either yourself, a friend or a private investigator follows them making very sure no signs of tailing them are present. If you are seeking physical proof a very current photo of them intimately is probably going to be necessary if you are presenting a legal position or trying to salvage your relationship.
Once your wife, husband, girl or boyfriend is caught the denial will turn into something else, could be that being caught makes you feel like you did something wrong. Suck it up, you didn't cheat, they did.