Adulteration in milk these days has become very common. There are many ways in which milk is adulterated. Most common and by far less harmful is adding water in the milk. Apart from impurities added from water, there are few dangers in consuming such a milk. To identify,adulteration from water, a simple test is milk slip test.viz, running the drop of milk on smooth or polished surface.Pure milk leaves some residue  or traces behind. However, milk mixed with water addition,will simply flow out. This is by far easy to know.
However, these days,adulteration is done in more ways than such a simple water adulteration.
For instance, very common form of adulteration  is Urea adulteration.
To identify this adulteration, do the following test.
Take a teaspoon of milk in a small cup. Add a teaspoon of soybean or arhar (Tur Dal) powder. Mix the contents thoroughly by shaking the mixture. After 5 minutes, dip a red litmus paper in it. Remove the paper after a minute. If the litmus paper shows change in color from red to blue, it indicates  presence of urea in the milk.
One more common form of adulteration is Artificial or synthetic milk.
This is one of the most dangerous form of milk adulteration. In order to identify this type,you may follow the process as below,-
Synthetic milk gives a  bitter after taste. It gives a ‘soapy’ feeling if it is rubbed between two fingers. Another indication is that,it  turns yellowish on heating. With some experience, one can easily make out whether it is a pure natural milk or a synthetic liquid sold as milk.