Deny, delete, minimise.
They will....
Deny it ever happened.
Delete any texts or emails.
Minimise what happened such as,
"No I haven't met anyone" to
"We only chatted online" then becomes
"We only met once"
Becomes
"We only kissed"
Becomes
"We only had sex once"
Or my favourite "we met once but I couldn't get it up"
really...? Oh well, that's alright then!
When you confront the other person they don't feel safe as you have discovered their secret, so they will either withdraw and not say anything, or become angry.
Very often they will turn it around to be your fault as you were too busy / tired after dealing with babies and toddlers all day and didn't give them enough attention sex
I often wonder what the response would be if we said " oh sorry darling, you were so busy working hard to keep us financially secure you were too tired to attend to my needs, so I shagged the plumber as he was pretty fit and up for it..."
Very often our instinct tells us something is amiss. IME your gut is usually right. If you're gut is telling you something's not quite right then you need to dig for evidence because if you just confront they will delete everything making you feel paranoid. You get this feeling because the other person has emotionally checked out of the relationship.
Hope you're ok OP