In multilingual Asian communities, determining language dominance for clinical assessment and intervention is often complex. The aim of this study was to develop a self-report classification tool for identifying the dominant language in English–Mandarin bilinguals. Participants (N = 168) completed a questionnaire on language history and single-word receptive vocabulary tests (Peabody Picture Vocabulary Test type) in both languages. The results of a discriminant analysis on the self-report data revealed a reliable three-way classification into English-dominant, Mandarin-dominant, and balanced bilinguals. The vocabulary scores supported these dominance classifications, whereas the more typical variables such as age of first exposure, years of formal instruction, and years of exposure exerted only a limited influence. The utility of this classification tool in clinical settings is discussed.