Filled milks (mixtures of vegetable oils and dairy proteins etc,) have long been used. Also, milkfat - vegetable oil blends are very widely used in spreads and artificial creams and baking.

This

Adding vegetable oils may or may not be an adulteration.As we know that milk or dairy products are rich in polyunsaturated fats which undergoes lipid oxidation.Sometimes, to limit or reduce the level of oxidation vegetable oils or essential oils are added.

In view of this, not all times vegetable oils can be an adulterant.

Vegetsble oil contains phytosterol but not cholesterol, presence of phytostrrol meaning sddition of vegetsble oli. You can extract fat from milk sample and analyse

You can analys extrscted fat using GC and/or physical properties