After spending time with someone for a number of months (or even years), you know their typical behavior. You know how they act in certain situations and when they act differently, you want an explanation—if not cheating, then something else real and tangible.
In this respondent's case, when he started his affair, he would grab his phone and head into the restroom for 20-30 minutes. His excuse: He was "reading email and the news." The only problem was, his phone bill showed something very different.
2. He has unpredictable mood swings.
Being angry in an instant—or being overly nice—is a red flag. There's no real reason for him to leave, so he has to create one.
New activities that he goes to all the time (but never mentions afterward) is a glaring example. If he was going somewhere and had no reason to hide it, he would most likely talk about it with you.
"I just need time alone."—while logical, excuses like these are usually without a reason or completely out of the norm for him.
3. He has a rapid change in weight or appearance.
Cheating men change physically. Their love handles disappear, their posture changes and their muscle tone/definition returns. It's the body's primal means of preparing for courtship—he's readying himself to attract a mate.
In addition, cheating men start taking much better care of themselves and take a lot of pride in their appearance. He's suddenly pressing his clothes, coiffing his hair and shaving daily when he didn't before ... especially when leaving the house without you.
4. He has an overnight change in musical taste.
There's nothing wrong with trying new things. But when it's out of the blue and he excludes you from the change, it's a warning sign that something is amiss.
5. He remembers things wrong or loses track of the details.
As he spends more time with both you and his mistress, the memories of his time spent with her and the memories of his time spent with you will start to blur together. Schedules get crossed and he won't be able to keep track of where he was on any given day. The time he spent with another woman is entirely unaccounted for—often, with no explanation.
6. He suddenly needs a lot of privacy.
To take time for one's self is normal. But when it's out of the normal pattern—or is more and more frequent—it's a sign that something else is going in. Going into work early, leaving late, getting off the phone suddenly, taking lots of private calls from a "relative" (mom, sister, someone untrackable) are also red flags.
7. He has a drastic change in sexual appetite and style.
A sudden shift in what's "regular" or familiar in bed—without discussing wanting something new—is a big warning sign of cheating. All the men I interviewed expressed experiencing a huge rush of testosterone, which manifested as a new confidence during sex.
They felt strong, masculine, virile ... like every woman wanted them. Their egos were growing daily.
8. He hides his financial history from you.
Most cheating men will need an untraceable way to pay for things: gifts, meals, hotels, trips, etc. Reloadable credit cards, PayPal accounts, eBay purchases are all great ways to hide large or frequent transactions. Definitely investigate large, random transfers or cash withdrawals.