Common signs that indicate a partner may be cheating include a sudden interest in improving his appearance, giving evasive or vague answers to direct questions, and a decrease in physical intimacy in the relationship. Secrecy is a big warning sign, as is time or money for which he can't account.
Unexplained changes in behavior might indicate an affair. A partner who suddenly begins dressing better or loses a lot of weight may be interested in someone new. New interests and hobbies that seem out of character may indicate that he is pursuing a new love interest. A cheating partner may be withdrawn and disinterested, or he may be unusually attentive in hopes of avoiding suspicion.
A partner who is cheating communicates in secret. He may become possessive of his phone, never letting it out of his sight. He may set up a new email account that his partner can't access, or even purchase a second phone. He steps outside or into another room to take phone calls.
Bank accounts and credit card statements show unexplained charges or cash withdrawals when a partner is having an affair. He may choose to travel alone rather than with his family, when in reality, he is traveling with his new love interest. He is gone for periods of time that he refuses to account for, or he gives vague explanations that cannot be verified.