Dissolve 1 g resorcinol in 330 mL concentrated HCl, dilute to one liter (approx. 4 M HCl final). This reagent seems to be stable for more than a year, though we usually make less than the recipe specifies.
When 1 mL of reagent is heated with 5 drops of sample in a boiling water bath, a positive test for ketoses (sucrose works, too) is formation of an orange to red color (not precipitate) within five minutes. Some sources say an apricot color is negative, but it's a judgment call. It depends on the concentration in the sample, and sugars like glucose give essentially no color even after ten minutes.
This reaction is also quantitative; absorbances can be read at around 480 nm. I haven't carefully documented the linear ranges in terms of amount of ketose and incubation times.