One limitation of the naturalistic observation method is that it is understudied how accurately personal relationships may be judged by observers in real-life settings. To assess this judgment accuracy, we observed 285 dyads of individuals in public places and then asked whether they were affiliated or strangers. We found that human observers were very accurate in judging peoples' actual personal relationships. Moreover, several nonverbal cues, including direct interaction and age similarities, were identified as correlates of affiliation. We conclude that researchers may accurately judge personal relationships from nonverbal observational data and recommend that this should be utilized as a methodological tool in naturalistic observational studies.