From Mises Wire:

Measuring gains or losses in the standard of living is not a simple matter. We know that over the past century in the United States, working hours have declined, and real incomes have increased. By nearly every measure we can imagine, the standard of living in the United States has increased substantially since the late nineteenth century and early twentieth century. But how have conditions changed over just the past 15 or 25 years? That’s a lot harder to draw conclusions about, and it’s hard to guess if today’s younger adults are going to be “better off” than their parents were at the same age.

There is no single number or metric we can point to that will tell us that “Americans are now X amount better off than they were in 1990.” 

Nevertheless, there are various measures we can investigate that can at least give us some insights