It's the one below the one ashwinsundar linked, "detection of artificial colour in turmeric powder". The test is: when you add the powdered turmeric to water,

It's the one below the one ashwinsundar linked, "detection of artificial colour in turmeric powder". The test is: when you add the powdered turmeric to water, natural turmeric will give the water a "light" yellow color, while adulterated turmeric will give it a "strong" yellow color. This is not a test that I'd characterize as "easy" or "reliable".