Let's first define what constitutes a "rhetorical device."
The Guide to Literary Terms by eNotes has an ample index with all the information you need to best understand how authors use their craft.
In the guide, the definition of "rhetoric" reads:
..the theory and principles concerned with the effective use of language or the theory and practice of eloquence, both written and oral. It consists of the rules that govern all prose composition or speech designed to influence the judgment or feelings of people
This said, a rhetorical device is a product rhetorical language. It is the use of words to cause a reaction, or make an effect, in the audience. This effect does not have to be some colossal reaction. Instead, rhetorical language effectively engages the imagination in a way that the reader can see and feel exactly (or close to) what the author sees and feels.
When reading Ethan Frome, the reader can feel sensations and create visuals that may or may not be part of their own life experiences. This tells you right away that the use of rhetorical devices is effective.
The most salient rhetorical devices in Ethan Frome are:
imagery, or using descriptive language to paint a vivid picture of what the author describes, complete with sounds, scents, sights, and sensations.
Anyone who has ever lived through long, cold winters may be able to understand the feeling of frigidity, stagnation, melancholy, boredom, and desperation that Edith Wharton infuses in her narrative as she describes the town of Starkfield. Even more frustrating is the fact that nothing in Starkfield is meant to flourish, particularly, in the family line of the Fromes.
As such, all the images you see in the novel are cold, sterile, and lackluster. Starkfield is not a winter wonderland, but far from it.
... I had been struck by the contrast between the vitality of the climate and the deadness of the community.
[...]a blazing blue sky poured down torrents of light and air on the white landscape, which gave them back in an intenser glitter. One would have supposed that such an atmosphere must quicken the emotions as well as the blood; but it seemed to produce no change except that of retarding still more the sluggish pulse of Starkfield.
similes, or comparisons between two or more things that are not typically comparable. Typically, a simile uses the prepositions "as" or "like" to establish the comparison.
We see the use of comparisons from the perspective of how Ethan felt with Mattie compared to how he feels with everything and everyone else.
Her wonder and his laughter ran together like spring rills in a thaw.
This example in chapter 2 shows that Ethan and Mattie's laughter felt like a frozen stream that starts to thaw, or melt slowly, due to a source of warmth. The warmth is the feeling of infatuation that Ethan feels for Mattie, and vice versa.
symbolism, or the use of objects, colors, and even characters, to represent ideas and other abstract thoughts.
Let's look no further than Zeena's red pickle dish. This pickle dish is Zeena's most beloved wedding present. It is the last vestige of what once was a normal marriage—perhaps one which was even passionate and hopeful. This is represented by the color red.
The fact that the dish is so fragile also symbolizes the state of the Frome's marriage. Lastly, the fact that Mattie took it without permission to use it with Ethan during a dinner behind Zeena's back supports the theory behind the symbolism of the dish. Moreover, Mattie's actions caused a chain reaction involving the cat, who caused the precious red dish to slip to the ground and come crashing down. When Zeena finds out, it is clear that she felt the loss of this dish quite deeply.
[Zeena gathered] up the bits of broken glass she went out of the room as if she carried a dead body...
This shows even more clearly that the dish symbolizes the impending crash of the relationship in itself.
For more information on how Edith Wharton used her rhetorical devices to create a place and a society like Starkfield, read the 1983 publication "Edith Wharton and Ethan Frome" by Orlene Murad. It was published by Modern Language Studies (vol 3. num. 3).