A simile is a direct comparison of two things that are unrelated but that do share one common characteristic or trait. The easiest way to identify a simile is by looking for the "noun like noun" or "noun/verb as a noun/verb" format.
Interestingly in "The Necklace", Maupassant chooses to describe the characters using direct and indirect characterization rather than using similes and other figurative language. Yet, an example of simile that is used in the narrative would be the following:
...she thought of delicious dishes served on marvellous plates and of the whispered gallantries to which you listen with a sphinx-like smile while you are eating the pink meat of a trout or the wings of a quail.
In this case, the smile is being directly compared to the facial expression of the sphinx. Since the comparison is direct, it is considered a simile.
Personification is the adjudication of human features to an inanimate or an inhuman subject. To spot an example of personification in literature, look for instances where objects are given some sort of power to dominate or control a human character. Often, authors use personification to show how the object resembles something that is alive.
In "The Necklace" we can find an example of personification in the following excerpt
...one of those ancient night cabs which, as though they were ashamed to show their shabbiness during the day, are never seen round Paris until after dark.
Since cabs do not have emotions, nor do they care whether they look shabby or not, the author has personified them by saying that they feel ashamed by the way that they look. This is a classic example of personification.