Style: The writing of Maupassant is typically regarded as being far simpler and more straightforward than that of Flaubert. Both writers were masters of their professions, but Maupassant was renowned for his ability to convey the essence of a situation or feeling in as few words as possible. His sentences often begin with verbs as opposed to subjects and are typically brief (even overly so). Flaubert, on the other hand, liked writing that was complicated and full of metaphors. His works are full of deep metaphors and precise details that are meant to create an environment full of subtleties that play on the readers' imaginations.
Literary Methods: On top of this difference in style, the two writers also had very different approaches when it came to literary methods like characterization and plot structure. Madame Bovary has become renowned worldwide for her careful focus on character psychology in particular, whereas Maupassant prioritized simplicity and plotted classic narratives, but he rarely dwells too long on psychological considerations, preferring instead to build suspense through understatement, leaving readers hanging until they can draw out underlying themes based on implications presented throughout each story without explicitly defining them outright.
Given a passage in French that you've never seen before, it may be difficult to tell, aside from contextual clues, who wrote it purely based on style or literary methods; however, if there are specific traits of the author’s craft, like short sentences used sparingly throughout the whole piece, then perhaps one could make an educated guess as to whose work had been presented to them given their intense familiarity with both authors.