The index of many differential/algebraic equations (DAEs) is determined by the structure of the system, that is, by the pattern of nonzero entries in the Jacobians. This paper considers an important subclass of DAEs which can be solved by backward differentiation methods if their index does not exceed two. For this reason, it is desirable to determine whether the index exceeds two or not. In this paper we present an algorithm that determines if the index is one, two, or greater, based only on the structure. The algorithm can be exponential in its execution time: we do not know whether it is possible to get an asymptotically faster algorithm. However, in many practical problems, this algorithm will execute in polynomial time.
