I have started to do some milk tests while making cheese as suggested by a master cheese maker. These tests are very simple to do and give a good indication of the quality of the raw milk.
A little milk without starter or rennet is poured into a sterilized test tube. The milk is warmed at 85 to 90 degree F and left at room temperature or near a heat source for 24 hours or more. I don't think it is necessary to keep the milk at the starting temperature for the entire time. In my experience, a curd will form at room temperature as well.
The formed curd will tell a lot about the quality of the milk. Of course, I am not an expert on this test. I just started doing it. I had some problems with some milk and I did not want to send milk to a laboratory for analysis all the time. Needless to say, the better and more homogeneous the formed curd, the better and cleaner the milk. One time, when I removed the rubber stopper from some tubes with milk after the curd had formed, some gas escaped. I clearly noticed a little puff. This was a clear indication of gas forming organisms present in the milk.