A guy might bathe or shower more often. He may take up running. He may start going to the gym or get himself a fashionable haircut. He may become more romantic within the marriage, or indeed he may become quite callous or indifferent towards you, and he will almost certainly start 'working late' or taking trips abroad on business. These things on their own do not prove a man is having an affair, but they are good indicators.
Women also change if they are having an affair.
They lose weight. They buy more modern clothes and get new hairstyles. They start 'going for a drink after work' and have more nights out, supposedly with girlfriends. They may take up an evening class. They will frequently buy their live-in partner little loving gifts because they are feeling guilty, and sometimes they become extra keen on sex and become more randy and hot in bed than they have for years.