You can do a simple lactic test with the milk at home. Warm the milk to about 70F /20C and put it in a sterilized test tube. Store this at room temperature until it is coagulated (about 1 to 2 days). The coagulum gives you an indication how clean the milk is. The more homogenize the curd, that is undisturbed curd; no cavities, no breaks, no gas formation etc means the milk is clean. The more disturbances in the curd, the dirtier the milk.