The main character was a maid and became a noble through marriage. In one chapter, she helps out some maids from the house she married into, and one of the maids snickers and makes fun of her because she is a nobody. When the house decides to make some of the maids her exclusive maids, the maids that she helped were concerned because they couldn't find the one that helped them and the one that made fun of her said "she didn't even get picked to be a candidate for the new lady". When the lady shows up, they get really shocked because she is the lady that helped them. She then picks them and the lady that made fun of her because that woman knows how to read and can help the maids that don't know how to read.
As confirmed by comments, this is The Villainess Wants to Enjoy a Carefree Married Life in a Former Enemy Country in Her Seventh Loop!
The Duke’s daughter, Rishe left the evening party without delay. In fact, for Rishe, this engagement has been terminated for the seventh time. ... Her past six lives have been busy and fulfilling, she wants to live longer this time around. If possible, she wants to live a life of leisure and squander money…. To commence her 7th life, Rishe has set up such a diagonal goal. However, in this life, she will be married to the Crown Prince of a militant nation, who killed her in her past life.
For the specific scene in the question, the main character is brought to her new castle, which was underused and still dirty. Her previous lives included living as a commoner and had plenty of practical skills, so she puts on a maid outfit and starts cleaning. Actual maids come in and don't realize she's the noble lady they'll be serving until some of them need to be selected to be personal servants.