Areader can usually recognize a sentence fragment because the reader is a. left asking questions. b. confused by too many words. c. not sure where details apply. d. puzzled where to stop reading.

A reader can usually recognize a sentence fragment because the reader is A. left asking questions.
A sentence fragment, as the name itself says, is incomplete - it has to be included into a larger sentence in order to make sense. For example: 'when he sleeps' is a fragment because we don't know what happens when he sleeps - there is no enough information.