From the initial stage directions, how do we know that the Birlings have an affluent lifestyle?

The play opens in the dining room of the Birlings's home, which the initial stage directions explain belongs to "a prosperous family." When performed on stage, the audience, of course, will not have the benefit of this stage direction, but the director of the play will be left in no doubt that the opening scene should clearly indicate the affluence of the family. This affluence is, after all, vital to one of the play's main moral messages, which, broadly, is that the concentration of wealth in the hands of a few is not conducive to a fair or moral society for the many.
The initial stage directions offer several ways in which the affluence of the Birling family might be indicated to an audience. The furniture, for example, should be "a good solid furniture of the period," and on the dining room table, there should be "champagne glasses" and a "decanter of port." The characters are also described as wearing "evening dress," with the men "in tails and white ties." These are not the clothes that a working or even middle class family would be wearing. The initial stage directions also introduce Edna, "the parlormaid," which again is a clear indication of a wealthy household. Altogether, the combination of the furniture, the clothes that the characters are wearing, and the presence of a parlormaid should leave an audience in no doubt that this is a very affluent, upper-class family.
The dining table and chairs are also, tellingly, described in the stage directions as being "center downstage." This dining table, of "good solid furniture," with its "champagne glasses," "decanter of port," and "dessert plates," is the symbol of upper class affluence. Priestley deliberately indicates that it should be placed at the front of the stage so that this symbol is very much in the foreground and the centre of attention. He also indicates, in the same stage directions, that the table should be "upstage" during act three, which is when the Birlingss' wealth is no longer the defining characteristic of the family but is replaced by their morality, or lack thereof. Accordingly, in act three, the dining table, the symbol of their wealth, is moved from the foreground to the background.

The stage directions at the opening of act 1 really couldn't be clearer in telling us about the Birlings' wealth and high social status. The Burlings live in a "large suburban house" belonging to a prosperous manufacturer. But there's nothing cozy or inviting about this place; it's substantial and heavily comfortable. One gets the impression that the Birling residence is designed more to impress other people than to provide a warm, homely living environment.
Additional evidence of the Birlings' affluence is provided by the stage directions. The family has a parlor maid, something that only the very wealthy would've been able to afford at that time. There are also champagne glasses on the table, a sure sign of the lavish lifestyle to which the Burlings have become accustomed. And to top it all off, everyone is wearing evening dress, with the gentlemen wearing white tails and ties.