Methods and apparatus to detect user attentiveness to handheld computing devices are disclosed. An example method includes detecting a change of a handheld computing device from a first starting spatial condition to a first ending spatial condition, wherein the first starting spatial condition and the first ending spatial condition are angular orientations of the handheld computing device relative to a reference; and querying an index associating a plurality of spatial condition changes with respective likelihoods indicative of user attentiveness to determine if the index includes an entry having the first starting spatial condition and the first ending spatial condition of the detected change.