To identify a writer’s style you have to look at what they focus on consistently, from story to story.
Steinbeck likes to write about common folks trying to make their way in a world that is often indifferent or even hostile. We see this with the Joad family in The Grapes of Wrath and with George and Lennie in Of Mice and Men.
Steinbeck also tends to focus on the importance of the natural world, providing a lot of descriptions of the land and its part in people’s lives. In The Grapes of Wrath he sometimes devotes significant parts of chapters to telling about what the land is like and how it has been affected by weather conditions (dust bowl). Of Mice and Men opens with a description of a peaceful patch of land in which we see the characters relaxed and at home for the only time in the story.
Finally, Steinbeck recreates the dialect of his characters with great detail. The way they talk tells the reader a lot about them. For example, in Of Mice and Men Lennie asks several times “Tell me, like you done before” when he wants to hear about their future plans. It shows what a simple, uneducated person is.