Although The Idiot is not a novel of manners, and so issues of class status and the subtle hierarchies of society are not a major concern, we do experience the differences between the aristocracy, the middle classes, and to a small extent the lower class. At the same time, the novel is concerned with society as a whole, and with its rules of behavior, which seem ironclad until someone demonstrates their bogus nature.
The characters who feel and care about class differences most strongly are those who suffer least from being exposed to these differences.
Although at least in part, the novel seems to study the way society works, in practice it is almost impossible to determine the social status of characters through their actions, speech, or even dress. For every slovenly peasant, there is an equally slovenly upper class character, and vice-versa.