How can you recognize cold milk that has gone bad or is about to?
Looks like FAO (Food and Agriculture Organization of the United Nations) publishes a formal testing handbook here.

One of the easier methods:


  2.4.5.3. The Alcohol Test
  
  The test is quick and simple. It is besed on instability of the
  proteins when the levels of acid and/or rennet are increased and acted
  upon by the alcohol. Also increased levels of albumen (colostrum milk)
  and salt concentrates (mastitis) results in a positive test.
  
  Procedure:
  
  The test is done by mixing equal amounts of milk and 68% of ethanol solution in a small bottle or test tube. (68 % Ethanol
  solution is prepared from 68 mls 96%(absolute) alcohol and 23 mls
  distilled water). If the tested milk is of good quality, there will be
  no coagulation, clotting or precipitation, but it is necessary to look
  for small lumps. The first clotting due to acid development can first
  be seen at 0.21-0.23% Lactic acid. For routine testing 2 mls milk is
  mixed with 2 mls 68% alcohol.


Simplified for home use:

This test is likely to be adaptable to household alcohol drinks such as vodka or similar beverages.  For example the number of teaspoons of Vodka an ounce or two of milk can hold before it clots.