Measure the concentration of lactose in dairy products I would like to measure the concentration of lactose in various dairy products : milk, cheese, yoghourt... 

How is it done in labs ?
How can I do that using cheap equipments ?

Thanks
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.