Measure the concentration of lactose in dairy products
If you are looking for economical methods then you have the option of titration and spectrophotometry. See this paper by Folin THE DETERMINATION OF LACTOSE J. Biol. Chem. 1918, 33, 521-524. The titration is based on the reducing ability of lactose.

Another reliable titrimetric method can be seen in this paper by Hites, Determination of Lactose in Milk Products J. Anal. Chem. 1949, 21(8), 993-995.