Provided stimuli are highly distinct, the detection of changes between two briefly separated arrays appears to be achieved by an all-or-none process where either the relevant information is in working memory or observers guess. This observation suggests that it is possible to estimate the average number of items an observer was able to retain across a series of trials, a potentially highly informative cognitive characteristic. For each version of the change detection paradigm, for this estimate to be accurate, it is important to specify how observers use the information available to them. For some instantiations of this task it is possible that observers use knowledge of the contents of working memory even when they are in a guessing state, rather than selecting between the response alternatives at random. Here we test the suggestion that observers may be able to use their knowledge of the number of items in memory to guide guessing in two versions of the change detection task. The four experiments reported here suggest that participants are, in fact, able to use the parameters of the task to update their base expectation of a change occurring to arrive at more informed guessing.