By taking nominal wages and then normalizing them to some specified year’s dollars using a price index. Yes I know you believe that price indices are made up but if anything CPI overstates inflation slightly because it doesn’t properly account for substitution across goods categories (for example if the price of apples increases people will buy fewer apples and more oranges, but the CPI won’t update the weights on apples and oranges immediately). That is why the Fed uses PCE as its target index, because it more accurately adjusts for substitution elasticities.