They test one sample of your milk before accepting it to make sure it is suitable for processing and they take other samples for laboratory tests. A bad smell or taste in the milk can be caused by bacterial growth, chemical reactions and by flavours being absorbed in the milk. Silage flavour is easily absorbed by the milk either directly or indirectly via the cow. However, if the silage is well-preserved and fed in reasonable quantities there will be no problem with silage flavour. Flavours from onions and certain weeds may be absorbed via the cow.
Unclean flavours are often due to unclean sheds and animals.
Keeping the milk too long in an open container in the milk shed may also cause unclean flavour. Medicine and chemical flavours usually come from medicine used to treat the cows and from detergents and disinfectants which have not been properly rinsed off the milking utensils. They look at your milk to check the colour and make sure that it is not lumpy. They smell your milk and taste it. Bad smell or taste shows low quality of milk.