We consider the problem of identifying the change formula in a belief revision scenario: given that an unknown announcement (a formula mu) led a set of agents to revise their beliefs and given the prior beliefs and the revised beliefs of the agents, what can be said about mu? We show that under weak conditions about the rationality of the revision operators used by the agents, the set of candidate formulae has the form of a logical interval. We explain how the bounds of this interval can be tightened when the revision operators used by the agents are known and/or when mu is known to be independent from a given set of variables. We also investigate the completeness issue, i.e., whether mu can be exactly identified. We present some sufficient conditions for it, identify its computational complexity, and report the results of some experiments about it.