We propose an approach for allowing users to assess the integrity of distributed queries computed by a computational cloud, which is trusted neither for data confidentiality nor for query integrity. In particular, we consider join queries over multiple data sources, maintained at separate (trusted) storage servers, where join computation is performed by an inexpensive, but potentially untrusted, computational cloud. We illustrate the working of our approach and its application in a MapReduce scenario. We also provide an analysis and experimental results.