  We discuss an approach for developing a diagnostic e-module, which can easily
identify the precise nature of deficiencies in conceptual understanding of a
topic, using Symmetry and Gauss's Law as an example. The topic is divided into
suitably chosen concept categories. A large number of True/False questions are
prepared for each concept category, which can be answered using the concepts in
that category. The module presents seven randomly chosen questions from each of
the concept categories. Only when each of the questions from a concept category
is answered correctly, will it be inferred that the concept category has been
understood properly. A diagnostic module based on this approach can reliably
and efficiently identify the conceptual deficiencies. After taking remedial
measures, the student can take the diagnostic test again till all conceptual
deficiencies are removed
