In the program checking paradigm, the original program is run on the desired input, and its output is checked by another program called a checker. Recently, the notion of program checking has been extended from its original formulation of checking functions to checking a sequence of operations which query and alter the state of an object external to the program, e.g., checking the interactions between a client and the manager (server) of a data structure. In this expanded paradigm, the checker acts as an intermediary between the client, which generates the requests, and the server, which processes them. The checker is allowed a small amount of reliable memory and may provide a probabilistic guarantee of correctness for the client. We present off-line and on-line checkers for data structures such as linked lists, trees, and graphs. Previously, the only data structures for which such checkers existed were random access memories, stacks, and queues.