This article is part of an ongoing project to develop a method for team-based learning named Testudo. We present an assessment technique called DuoTest, which allows students to do their final exam twice in a row: the first time, participants do their exam individually (Exa01); the second time, they solve the same exam in groups (Exa02). By comparing individual and group exams, the system induces the positive (or negative) effect of each team over the individual performances. Empirical results collected from 70 students show that individual exams are a reliable, although weak, predictor of the group scores (p<0.10, Adj R2= 0.02). Instead, by measuring the fixed effect of each team, we obtain a better predictor of Exa02 (Adj R2= 0.71). Although additional testing is required, our guidelines address a current gap in the literature for techniques that rigorously assess the individual and team dimensions, and that are easy to implement.