Do humans have an intuitive understanding of whom they can trust? Research on the accuracy of trustworthiness detection abilities has produced mixed results. However, most studies have limited themselves to trustworthiness detection from static photographs of strangers or short videotapes and ignored interactions between people that already know each other. Thus, we investigated trustworthiness detection in situations that more closely resemble real-life trust interactions. In seven studies involving 580 participants and about 13.000 individual trust game interactions we found that trustworthiness detection was inaccurate among strangers either after watching short video-clips of potential trustees or after short personal interactions during a group task with potential trustees. In contrast, trustworthiness estimates were better than chance among already acquainted participants. Furthermore, we found evidence that people successfully use the relationship quality they have with a potential trustee as a “fast and frugal heuristic” when estimating another person’s trustworthiness.