Differentiating bilingual children with primary language impairment (PLI) from those with typical development in the process of learning a second language has been a challenge. Studies have focused on improving the diagnostic accuracy of language measures for bilinguals. However, researchers are faced with two main challenges when estimating the diagnostic accuracy of new measures: (a) using an a priori diagnosis of children (children with and without PLI), as a reference may introduce error given there is no gold standard for the a priori classification; and (b) classifying children into only two groups may be another source of error given evidence that there may be more than two language ability groups with different strengths and weaknesses or, alternatively, a single group characterized by a continuum of language performance. The current study tested for the number of distinct language ability groups and their characteristics in predominately Spanish-speaking children in the U.S. without using an a priori classification as a reference. In addition, the study examined to what extent the latent groups differed on each measure, and the stability of language ability groups across three assessment methods in Spanish (standardized tests, language sample analyses, and comprehensive assessment), taking in to account English and non-verbal cognitive skills. The study included 431 bilingual children attending English-only education. Three latent profile analyses were conducted, one for each method of assessment. Results suggested more than two distinct language ability groups in the population with the method of assessment influencing the number and characteristics of the groups. Specifically, four groups were estimated based on the comprehensive assessment, and three based on standardized assessment or language sample analysis in Spanish. The stability of the groups was high on average, particularly between the comprehensive assessment and the standardized measures. Results indicate that an a priori classification of children into two groups, those with and without PLI, could lead to misclassification, depending on the measures used.