Examining milk for presence of harmful bacteria
A simple way to know if milk contains any bacterial contaminations is to take ~5 ml of the milk and add a couple of drops of methylene blue to it. If the milk is sterile, it will stay blue. If any bacteria are present they tend to take up the dye, leaving the sample colourless after ~1 hour. A more extensive way would be to plate 3-5 dilutions of the milk sample on LB or nutrient agar.
You don't have to do much extensive sampling. Just make sure that you take a couple of well-stirred samples.