In this research, we provide two important contributions to the psychiatric community. First, we offer the first valid and reliable cognitive test that measures forensic clinicians' ability to detect and avoid diagnostic biases in psychiatric assessments. Second, we also estimate the prevalence of clinical decision bias detection and prevention ability among psychiatrists and psychologists. A total of 1069 clinicians from different specialties (317 psychiatrists and 752 clinical psychologists, of which 286 were forensic clinicians) participated in this research. The Checklist of Biases for Clinicians (BIAS-31) was developed, and its psychometric properties were analyzed. The prevalence of bias detection and prevention was estimated using BIAS-31 scores. The BIAS-31 is valid and reliable for measuring clinicians' ability to avoid and detect clinical biases. Between 41.2% and 55.8% of clinicians try to avoid biased clinical judgments. Likewise, between 48.5% and 57.5% of clinicians were able to correctly detect the biases involved in the diagnostic assessment process. We did not expect to obtain these prevalences. Therefore, we discuss to what extent specific training in the prevention of diagnostic biases is necessary and propose several clinical strategies to prevent a priori the occurrence of biases in the psychiatric assessment.