How do you know when your partner is having an affair?

good ?. My guess would be firstly lack of involvement in family life. Maybe an anxiousness or the appearance of nervousness around the home. Little lies about little things that don't make sense to the other person. Making up excuses for lateness. Gong to the 'store' 3 or 4 times instead of making one trip. It really depends on the person and how much extra unaccounted for time they have.