How does Steinbeck show what the living conditions are like on the ranch?

First, Steinbeck offers a detailed description of the bunk house in which the ranch hands live: an unpainted floor, eight bunkbeds, apple boxes nailed to the walls to act as shelves for personal belonging, a wood stove to heat the house. We see that the living conditions are spartan. The men do not have private rooms or even private spaces and the bunk house contains no luxuries. 

We also learn about living condition through conversation...

First, Steinbeck offers a detailed description of the bunk house in which the ranch hands live: an unpainted floor, eight bunkbeds, apple boxes nailed to the walls to act as shelves for personal belonging, a wood stove to heat the house. We see that the living conditions are spartan. The men do not have private rooms or even private spaces and the bunk house contains no luxuries. 

We also learn about living condition through conversation and action: George learns that Whitey, a previous hand, said he left because of the food. George carefully examines his mattress for bedbugs and lice, because Whitey left behind a powder to kill bugs, including roaches. This would indicate that such infestations were not uncommon.

Finally, we witness the boss, Curley, as he enters the bunk house and speaks threateningly to George and Lennie, warning them not to try to put anything over on him.

Everything in this initial description of living conditions indicates that ranch hands live the harsh lives of people at the lower end of the economic scale.