Some signs of a cheating boyfriend include when the boyfriend takes his phone with him everywhere, including to the bathroom, and when the man resists or refuses to introduce the girlfriend to his male friends, despite having dated his girlfriend for a significant period of time. If the man accuses his girlfriend of acting too clingy or paranoid when the girlfriend asks about the man's suspicious behavior, it may be another sign of a cheating boyfriend.
Another sign that a boyfriend is having an affair is if the boyfriend suddenly becomes uncharacteristically prompt or dependable. The boyfriend might try to hide his behavior by calling at a certain time each day or arriving home from work at a certain time. If his typical pattern recently changed it could be a sign that he is unfaithful.
Any change in typical behavior is an indicator that a boyfriend might be cheating. For example, if the boyfriend begins to use sayings or catch phrases that he did not say in the past, he might have learned the phrases from a new romantic interest. Alternatively, encouraging his girlfriend to flirt with strangers might be a way that a boyfriend handles his feelings of guilt over his own unfaithfulness.
If the boyfriend is upset or reacts poorly when the girlfriend picks up his phone, that may be another indicator of infidelity. If the boyfriend winces if his girlfriend uses his phone to do something innocuous such as look at the time or find directions, he might have something to hide. Hiding sudden unexplained bruises or other physical remnants of his misbehavior is also a sign of a cheating boyfriend.