People frequently adopt extra-pair mating strategies, which could be potentially harmful for their legitimate partners. In order to protect themselves from the costs of cheating, people need first to detect infidelity, and for this purpose, they employ specific infidelity-detection strategies. By using a combination of qualitative research methods, we identified 47 acts that people perform in order to detect their partners’ infidelity. Using quantitative methods, we classified these acts into six broader strategies for detecting infidelity. Participants indicated that they were more likely to employ the “Observe changes in her/his behavior,” followed by the “Ask and observe her/his reactions,” and the “Check where she/he is” strategies. Almost 58% of the participants indicated that they would use three or more strategies in order to detect their partners’ infidelity. We also found that higher scorers in Machiavellianism and psychopathy were more likely to employ the identified strategies than lower scorers. In addition, sex and age effects were found for most strategies.