1. A method of identifying a source of data input to a computing system comprising: a) receiving speech utterance from an entity related to randomly selected challenge text; wherein said challenge text represents a selected set of one more contiguous words which when articulated have a measurable difference in acoustical characteristics between a reference human voice and a reference computer synthesized voice that exceeds a target threshold; further wherein said challenge text is also selected in part based on a confirmation from a human listener that an articulation of such challenge text by a computer synthesized voice is distinguishable from an articulation of such challenge text by a human voice; b) processing said speech utterance with said computing system to compute first acoustical characteristics of said entity; and c) generating a determination of whether said speech utterance originated from a machine or a human.